x.py check --stage 1 silently just checks bootstrap (stage0) compiler

[CAD97]

./x.py check --stage 1 does not build anything (yay, check without waiting for a build!), which means that it cannot actually be checking the stage1 (post-bootstrap) build, and is instead just silently checking the stage0 (bootstrap) build.

This would be most useful with ./x.py check --stage 1 --keep-stage 0, as this would keep this as a check-only build rather than requiring a build of stage 0. This unfortunately can't really be automatic, as changes to the compiler that necessitate a check run would prompt a rebuild of stage 0.