GCCBootstrap-v5.2.0+6: [Rootfs] Teach testsuite to be less headstrong about `CC`, `CXX` etc...

The testsuite should do what the comment says; it should not assume
values for `CC`, `CXX`, etc... this manifested as an incorrectly-set
`RUSTC`, which should not have been a problem in the first place, but
still is sub-optimal