Auto merge of #125426 - jieyouxu:rmake-support-env-reset, r=<try>

Update `compiler-builtins` test to not clear essential env vars

Noticed in #122580 (comment), the `compiler-builtins` test failed on Windows for a `cargo` invocation because necessary env vars `TMP` and `TEMP` were cleared by `Command::env_clear`, causing temp dir eventually used by codegen to fallback to the Windows directory, which will trigger permission errors.

This PR adds a `clear_non_essential_env_vars` helper, which is a more conservative `Command::env_clear` that does not clear `TMP` or `TEMP` on Windows, and does not clear `TMPDIR` on non-Windows platforms.

cc `@ChrisDenton` do you happen to know if there are any more "essential" env vars that we should not clear (on Windows or other platforms)?

r? `@saethlin` (feel free to reroll, since you authored the test)

try-job: x86_64-msvc
try-job: test-various

Author: bors

tests/run-make/compiler-builtins/rmake.rs

@@ -56,7 +56,8 @@ fn main() {
         "--target",
         &target,
     ])
-    .env_clear()
+    // FIXME(jieyouxu): so what happens if we don't `env_clear`?
+    //.env_clear()
     .env("PATH", path)
     .env("RUSTC", rustc)
     .env("RUSTFLAGS", "-Copt-level=0 -Cdebug-assertions=yes")