Tracking Issue for decoupling from github actions environment variables in bootstrap

[albertlarsan68]

This is a tracking issue for the decoupling from github actions environment variables in bootstrap.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter.

Steps

• Implement the decoupling (cc @rust-lang/bootstrap -- can anyone write up mentoring
  instructions?)

The problem

See #108959, #102428 (comment) and #102428 (comment) for informations and what to do.

Unresolved Questions

• None at this time

Implementation history

• A first try in Add rust-lang CI environment check #102428, but got inactive

[jyn514]

This seems fine to do, but a lot of work and very hard to test.