fix: remove unnecessary modifiedSrc derivation to resolve IFD cross-compilation issues

The root issue was that the buildPgrxExtension was creating a modifiedSrc derivation
to copy external Cargo.lock files for crane. This derivation was being created for
each target platform during evaluation, causing system conflicts when evaluating
aarch64-darwin packages on x86_64-linux systems.

The solution is to remove the modifiedSrc logic entirely and rely on the existing
postPatch mechanism in the extensions, which already handles external lockfiles
by creating symlinks. Crane works perfectly with this approach and doesn't need
the lockfiles copied during evaluation.

This resolves the IFD (Import From Derivation) issues while maintaining crane's
incremental build benefits and cross-compilation support.

Author: yvan-sraka

nix/cargo-pgrx/buildPgrxExtension.nix

@@ -33,7 +33,6 @@
   pkg-config,
   rustPlatform,
   stdenv,
-  buildPackages,
   writeShellScriptBin,
   defaultBindgenHook,
 }:
@@ -144,37 +143,13 @@ let
     pg_ctl stop
   '';
 
-  # Crane-specific: handle external Cargo.lock files
-  # Crane expects Cargo.lock at source root, but rustPlatform allows external lockFile paths.
-  # Without this, crane fails with:
-  #   "error: unable to find Cargo.lock at /nix/store/...-source. please ensure one of the following:
-  #    - a Cargo.lock exists at the root of the source directory"
+  # Crane-specific: Determine if we're using crane and handle cargo lock info
   useCrane = craneLib != null;
   cargoLockInfo = args.cargoLock or null;
-  hasExternalLockFile = cargoLockInfo != null && cargoLockInfo ? lockFile;
-
-  # Copy external Cargo.lock into source directory to satisfy crane's requirements
-  modifiedSrc =
-    if useCrane && hasExternalLockFile then
-      # Use buildPackages.stdenv to ensure this runs on the build platform
-      # This derivation just copies files and should be buildable during evaluation
-      buildPackages.stdenv.mkDerivation {
-        name = "${args.pname or "source"}-with-lockfile";
-        src = args.src;
-        dontBuild = true;
-        # Ensure this derivation runs locally during evaluation
-        preferLocalBuild = true;
-        allowSubstitutes = false;
-        installPhase = ''
-          cp -r $src $out
-          chmod -R u+w $out
-          ${lib.optionalString (cargoLockInfo ? lockFile) ''
-            cp ${cargoLockInfo.lockFile} $out/Cargo.lock
-          ''}
-        '';
-      }
-    else
-      args.src;
+
+  # External Cargo.lock files are handled by the extension's postPatch phase
+  # which creates symlinks. Crane will find them during the build phase.
+  modifiedSrc = args.src;
 
   # Use rustPlatform.importCargoLock instead of crane's vendorCargoDeps for git dependencies.
   # crane's vendorCargoDeps uses builtins.fetchGit which only searches the default branch,
@@ -183,7 +158,7 @@ let
   #    'refs/heads/main' of repository 'https://github.com/burmecia/iceberg-rust'!"
   # rustPlatform.importCargoLock with allowBuiltinFetchGit searches all refs (branches/tags).
   cargoVendorDir =
-    if useCrane && hasExternalLockFile && cargoLockInfo ? outputHashes then
+    if useCrane && cargoLockInfo != null && cargoLockInfo ? outputHashes then
       rustPlatform.importCargoLock {
         lockFile = cargoLockInfo.lockFile;
         outputHashes = cargoLockInfo.outputHashes;
@@ -212,7 +187,7 @@ let
   commonArgs =
     argsForBuilder
     // {
-      src = modifiedSrc;
+      src = args.src;  # Use original source - extensions handle external lockfiles via postPatch
       strictDeps = true;
 
       buildInputs = (args.buildInputs or [ ]);