Run test --stage 0 library/std in CI

[jyn514]

These tests were recently broken by #95449. It shouldn't be possible for these to be broken on master - nearly everyone who contributes to the standard library uses --stage 0, and this makes it very hard to verify changes.

We should get these running in mingw-check or some other builder.

@rustbot label +P-high +A-testsuite +T-libs

Originally posted by @Mark-Simulacrum in #95993 (comment)

[gimbling-away]

@rustbot claim

[gimbling-away]

cc @pietroalbini Any idea re this? :)

[jyn514]

@gimbles here's where the code for mingw check lives:

rust/src/ci/docker/host-x86_64/mingw-check/Dockerfile

Lines 34 to 44 in a8ede12

	ENV SCRIPT python3 ../x.py --stage 2 test src/tools/expand-yaml-anchors && \
	python3 ../x.py check --target=i686-pc-windows-gnu --host=i686-pc-windows-gnu --all-targets && \
	python3 ../x.py build --stage 0 src/tools/build-manifest && \
	python3 ../x.py test --stage 0 src/tools/compiletest && \
	python3 ../x.py test --stage 2 src/tools/tidy && \
	python3 ../x.py doc --stage 0 library/test && \
	/scripts/validate-toolstate.sh && \
	/scripts/validate-error-codes.sh && \
	# Runs checks to ensure that there are no ES5 issues in our JS code.
	es-check es6 ../src/librustdoc/html/static/js/*.js && \
	eslint -c ../src/librustdoc/html/static/.eslintrc.js ../src/librustdoc/html/static/js/*.js

that job is also run on a full merge (bors r+) so it should be enough to only modify that.

[gimbling-away]

Awesome, thanks!

[gimbling-away]

Should be resolved by #99040