Add default GitHub Actions environment variable as default passthru

[louis-bompart]

Goals

1. As a preexisting user of GitHub Actions, I want to onboard onto turborepo without having to "decipher" GitHub logic, or Turborepo logic

Non-goals

1. It doesn't try to address the fundamental of this issue, which is that it's very tough to debug/understand what environment variable turborepo might be obfuscating if you are not in full control of the runtime.

Background

There is precedent for adding default passthru: #9235

Proposal

See #10740

[anthonyshew]

Merged, thanks!