Update `compiler-builtins` test to not clear essential env vars
Noticed in https://github.com/rust-lang/rust/pull/122580#issuecomment-2125755689, 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 removes the `env_clear` on the cargo invocation.
r? `@saethlin` (feel free to reroll, since you authored the test)
try-job: x86_64-msvc
try-job: test-various
23e040adec