Rollup merge of #81064 - Mark-Simulacrum:support-stage1-check, r=jyn514

Support non-stage0 check

Seems to work locally - a full stage 1 check succeeds, building std (because we can't get away with checking it), and then checking the compiler and other tools. This ran into the problem that a unconditional x.py check in stage 1 *both* checks and builds stage 1 std, and then has to clean up because for some reason the rmeta and rlib artifacts conflict (though I'm not actually entirely sure why, but it doesn't seem worth digging in in too much detail).

Ideally we wouldn't be building and checking like that but it's a minor worry as checking std is pretty fast and you can avoid it if you're aiming for speed by passing the compiler (e.g., compiler/rustc) explicitly.

r? ```@jyn514```

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