bootstrap: Remove `rustc_snapshot_libdir` from PATH in one more place Same as https://github.com/rust-lang/rust/pull/141657 but in a different part of the build system, with the same goal of addressing [#t-infra/bootstrap > Build broken in MSYS2 @ 💬](https://rust-lang.zulipchat.com/#narrow/channel/326414-t-infra.2Fbootstrap/topic/Build.20broken.20in.20MSYS2/near/520709527). It seems to work on windows-{gnu,msvc} and linux-gnu at least. r? jieyouxu