Bootstrap deletes the sysroot directory when alternating between check and build, breaking --keep-stage

[davidtwco]

If I run x.py build without --keep-stage or --stage and then in subsequent builds use only --keep-stage 0 then bootstrap runs as expected.

However, it seems though that if I switch to using x.py check (still with --keep-stage 0) then any subsequent x.py builds with --keep-stage 0 will fail with the below error unless I do a full build without --keep-stage 0 again.

thread 'main' panicked at 'File::open(stamp) failed with No such file or directory (os error 2)', bootstrap/lib.rs:1109:12
note: Run with `RUST_BACKTRACE=1` for a backtrace.

[jyn514]

I'm going to close this as a duplicate of #34279, which has some more discussion.