Enforce in bootstrap that check must have stage at least 1

[Kobzol]

This PR is another step towards https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Proposal.20to.20cleanup.20stages.20and.20steps.20after.20the.20redesign/with/523586917, this time dealing with x check.

It enforces the invariant that:

• We check std stage N with rustc stage N
• We check everything else stage N with rustc stage N - 1

It creates a single function that prepares a proper build compiler for checking something, and also adds snapshot tests for various common check steps. Some obsolete code was also removed.

The default check stage also becomes 1, for all profiles. I tested manually that x check std with download-ci-rustc still works and doesn't build rustc locally.

Subsumes #139170.

r? @ghost

[Kobzol]

CC @jieyouxu (one of many heaps of bootstrap fixup commits I have in store 😆)

[bors]

☔ The latest upstream changes (presumably #141899) made this pull request unmergeable. Please resolve the merge conflicts.

[Kobzol]

Cross-compilation checks are now doing too much work, I need to take a look at how we can reduce that, because x check --stage1 with cross-compilation is no longer a thing.

[rust-log-analyzer]

The job mingw-check-1 failed! Check out the build log: (web) (plain)

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#16 2.873 Building wheels for collected packages: reuse
#16 2.874   Building wheel for reuse (pyproject.toml): started
#16 3.090   Building wheel for reuse (pyproject.toml): finished with status 'done'
#16 3.091   Created wheel for reuse: filename=reuse-4.0.3-cp310-cp310-manylinux_2_35_x86_64.whl size=132719 sha256=d2a2565e7037ad3883fb9337653f2e25bbb588534fbef3697286cbc26d1bf634
#16 3.091   Stored in directory: /tmp/pip-ephem-wheel-cache-dy_04mdo/wheels/3d/8d/0a/e0fc6aba4494b28a967ab5eaf951c121d9c677958714e34532
#16 3.093 Successfully built reuse
#16 3.093 Installing collected packages: boolean-py, binaryornot, tomlkit, reuse, python-debian, markupsafe, license-expression, jinja2, chardet, attrs
#16 3.494 Successfully installed attrs-23.2.0 binaryornot-0.4.4 boolean-py-4.0 chardet-5.2.0 jinja2-3.1.4 license-expression-30.3.0 markupsafe-2.1.5 python-debian-0.1.49 reuse-4.0.3 tomlkit-0.13.0
#16 3.495 WARNING: Running pip as the 'root' user can result in broken permissions and conflicting behaviour with the system package manager. It is recommended to use a virtual environment instead: https://pip.pypa.io/warnings/venv
#16 DONE 3.6s