stage0 run-make compiletest broken

[bjorn3]

COMPILETEST_FORCE_STAGE0=1 ./x.py test --stage 0 tests/run-make no longer works. librun_make_support.rlib is compiled using the bootstrap rustc wrapper which sets --sysroot build/$HOST_TRIPLE/stage0-sysroot, but then compiletest will compile rmake.rs using the sysroot of the bootstrap compiler causing it to not find the libstd.rlib against which librun_make_support.rlib is compiled.

This regressed with #113026. (cc @jieyouxu)

[bjorn3]

See also https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/stage0.20compiletest.20broken

[jieyouxu]

From the zulip comment

Pass the right --sysroot when compiling rmake.rs, use the bootstrap rustc wrapper in compiletest too? Or something else?

Passing the right --sysroot might be the least disruptive solution (would anything wonky happen if we use bootstrap rustc wrapper in compiletest too?), but I'm not sure if this is the best solution.