fix: Preserve TMPDIR in strict environment mode (#14970)
### Description
Fixes #14965.
Strict environment mode already preserves `TMP` and `TEMP`, but omitted
`TMPDIR`, the temporary-directory variable used by Unix/Linux tooling
including
Node's `os.tmpdir()`.
This adds `TMPDIR` to `BUILTIN_PASS_THROUGH_ENV`. The regression test
verifies
that `TMP`, `TEMP`, and `TMPDIR` pass through while an unrelated
variable stays
filtered, preserving strict-mode semantics.
### Testing Instructions
- `cargo test -p turborepo-env`
- `cargo test -p turborepo-task-hash`
- `cargo clippy -p turborepo-env --all-targets -- -D warnings`
- `cargo fmt --all -- --check`
The broader task-hash test was run with Zig 0.17.0, matching the
repository's
`.github/actions/setup-zig/action.yml` default.
Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
