run-make: extract bootstrap cargo invocations to a proper `Command`

[jieyouxu]

Current bootstrap cargo incantations are nothing short of magic and are indecipherable. We should try to refactor them to proper Command wrappers.

let bootstrap_cargo = env_var("BOOTSTRAP_CARGO");
let mut cmd = cmd(bootstrap_cargo);
cmd.args(&[
    "build",
    "--manifest-path",
    manifest_path.to_str().unwrap(),
    "-Zbuild-std=core",
    "--target",
    &target,
])
.env("PATH", path)
.env("RUSTC", rustc)
...

[lolbinarycat]

@rustbot claim

[lolbinarycat]

any idea why these are explicitly inheriting environment variables? it's not like they call env_clear or anything...

[jieyouxu]

Sorry I didn't notice this issue, it's because at the time when I wrote run-make infra I just copied whatever the previous Makefile implementation did: it simply passed the already-available bootstrap cargo to run-make tests.

As discovered during a beta backport we found out that run-make -Z build-std tests fail on compiler_builtins on older branches #130634 because we added some new Cargo.lock setup that the bootstrap cargo did not know how to handle, but which is present in a cargo that is built from sources by a stage1 rustc.

This particular issue is fixed by a combination of #130642 and #130739 where we properly ensuring a suitably-staged cargo in bootstrap is available, and passing a path to that cargo in compiletest (see #130739 for details). We removed any BOOTSTRAP_CARGO in the test suite, run-make-support and compiletest.