[build] fix make prepare on clean agents: empty $(PkgMono_Unix)

[jonathanpeppers]

Summary

The Xamarin.Android Nightly build fails in make jenkins -> make prepare:

/Users/builder/.dotnet/sdk/10.0.302/Microsoft.Common.CurrentVersion.targets(5397,5):
  error MSB3030: Could not copy the file "/runtimes/linux-x64/native/libMono.Unix.so"
  because it was not found.
  [.../build-tools/Xamarin.Android.Tools.BootstrapTasks/Xamarin.Android.Tools.BootstrapTasks.csproj]

The leading bare / shows $(PkgMono_Unix) evaluated to an empty string. The package content is present; the path property was simply never set.

Root cause

$(PkgMono_Unix) comes from GeneratePathProperty="true" on the Mono.Unix PackageReference in MSBuildReferences.projitems. NuGet writes it into obj\*.nuget.g.props, which only a fresh project evaluation picks up. build-tools/scripts/Prepare.proj invoked BootstrapTasks with:

<MSBuild Projects="...BootstrapTasks.csproj" Targets="Restore;Build" />

The MSBuild task evaluates a project once and runs both targets against that single project instance, so unlike the -restore switch it never re-evaluates between Restore and Build. With no pre-existing obj/, the static None item in MonoUnixNative.targets therefore resolved to a root-relative path.

Before 94e4833 the prepare target ran dotnet build Xamarin.Android.BootstrapTasks.sln, whose implicit restore does re-evaluate — which is why this only regressed now, and only on clean agents. A warm obj/ on a dev machine hides it.

Fix

Build these projects out of process with dotnet build, matching the approach in #12199. The change to Prepare.proj here is identical to that PR's, so the two will not conflict. dotnet build restores in a separate evaluation and also writes a .binlog per project, so the next make prepare failure is diagnosable from CI artifacts.

PrepareOpenJDK in DotNet.targets gets the same treatment, since it had the identical latent bug.

Also adds _ValidateMonoUnixPackagePath to MonoUnixNative.targets so an unset $(PkgMono_Unix) reports what is actually wrong rather than an MSB3030 on a nonsense path. It runs BeforeTargets="PrepareForBuild" so it fires before _CopyOutOfDateSourceItemsToOutputDirectory can attempt the copy.

Testing

Verified locally with a clean bin/ and obj/:

• The old Targets="Restore;Build" pattern reproduces the failure — D:\runtimes\... on Windows, the same empty-prefix bug as CI's /runtimes/....
• With the guard in place, that same pattern now reports only:

  error : `$(PkgMono_Unix)` is not set. Build this project with `dotnet build` so
  NuGet-generated props are evaluated after restore.
  

• dotnet build of BootstrapTasks succeeds with 0 warnings and 0 errors, and copies the real libMono.Unix.so (120768 bytes, matching the file in the Mono.Unix package) plus libMono.Unix.dylib.

[Copilot]

Pull request overview

Fixes a clean-agent make prepare failure where NuGet GeneratePathProperty values (notably $(PkgMono_Unix)) were unset during build because restore/build were executed in a single MSBuild project evaluation.

Changes:

• Replace in-proc <MSBuild Targets="Restore;Build"> invocations with out-of-process dotnet build in Prepare.proj for BootstrapTasks and workloads.
• Apply the same out-of-process dotnet build approach to PrepareOpenJDK in DotNet.targets.
• Add an early MSBuild guard target in MonoUnixNative.targets to fail with a clear error when $(PkgMono_Unix) is not set.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
build-tools/scripts/Prepare.proj	Switches BootstrapTasks/workloads builds to dotnet build so NuGet path props are available on clean agents.
build-tools/scripts/MonoUnixNative.targets	Adds an explicit validation target to error clearly when $(PkgMono_Unix) is missing.
build-tools/scripts/DotNet.targets	Updates OpenJDK preparation to use dotnet build for the same restore/evaluation correctness.

[jonathanpeppers]

Verified on a real nightly run

Queued Xamarin.Android Nightly (definition 14072) against this branch: build 14762051 at 7e64dfe8.

make jenkins and macOS > Create Installers both succeeded. From the CI log:

Prepare:
  ".../bin/Release/dotnet/dotnet" build ".../Xamarin.Android.Tools.BootstrapTasks.csproj" \
    -p:Configuration=Release -bl:.../msbuild-20260724T112106-bootstrap-tasks.binlog
    Restored .../Xamarin.Android.Tools.BootstrapTasks.csproj (in 940 ms).
    Xamarin.Android.Tools.BootstrapTasks -> .../bin/BuildRelease/net10.0/Xamarin.Android.Tools.BootstrapTasks.dll
  Build succeeded.
      0 Warning(s)
      0 Error(s)
  BinaryLogger wrote to: .../msbuild-20260724T112106-bootstrap-tasks.binlog

Zero occurrences of MSB3030 in the log, and libMono.Unix.so now copies from the real package path (/Users/builder/.nuget/packages/mono.unix/7.1.0-final.1.21458.1/runtimes/...) instead of the root-relative /runtimes/.... The per-project binlogs land as expected.

Stage-level before/after

Stage	main 14755074	14759563	this PR 14762051
Build	failed	failed	succeeded
Test APKs	skipped	skipped	failed
Test Localization	skipped	skipped	failed
Test TimeZoneInfo	skipped	skipped	failed

Both prior nightlies died at make jenkins, so every downstream stage was skipped. This is the first run where Build goes green and the pipeline actually reaches the test stages.

About the downstream failures

Unrelated to this change — emulator infrastructure:

error : Unexpected output of 'pm list features' command: 'cmd: Can't find service: package'
error XABAS0000: com.android.tools.build.bundletool...

bundletool is querying an emulator whose package service isn't up yet. It retried 3x and failed. These stages have been skipped for at least the last four nightlies, so there is no green baseline to compare against — they were simply never reachable before. Out of scope here.