Rollup merge of #138743 - onur-ozkan:override-is-ci-behaviour, r=Kobzol

bootstrap: add `--ci` flag

To make bootstrap act like it's running on CI, we had to override the `GITHUB_ACTIONS` environment variable which is a hidden detail of `CiEnv::is_ci`. Now, we can use the `--ci` flag directly on bootstrap which will be documented automatically from `x --help`. This also helps us to avoid race conditions on bootstrap (overriding `GITHUB_ACTIONS` env in each test can cause that if we run the tests in parallel) tests.

Author: matthiaskrgr