bootstrap sets `-Zunstable-options` on "check" but not "build", leading to build script reruns

[RalfJung]

When working on Miri I noticed "portable-atomics" gets rebuilt every time I switch between a check-build and actutally running tests. This boils down to RUSTFLAGS being different between the two invocations: in check builds it contains -Zunstable-options but in test builds it does not.

[RalfJung]

Cc @rust-lang/bootstrap

[onur-ozkan]

HEAD: 7cc997d

Seems working fine here.

[RalfJung]

I'll have to try if I can till reproduce. But note that I mentioned "test build" above, so this would be ./x.py check miri vs ./x.py test --stage 0 miri.

[RalfJung]

Yeah I can still reproduce the problems with those invocations. (It's not the latest master but recent enough: 3089c31.) Sorry for not giving the exact commands in the issue description.

[onur-ozkan]

gets rebuilt every time I switch between a check-build and actutally running tests

I missed the tests part, sorry.

I just reproduce it too. I will be working on it by the coming week.

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