fix: let test-namespace-darwin use the hermetic cc toolchain

[basvandijk]

CI sometimes runs into the following error on the workflow: Bazel Test macOS Apple Silicon:

ERROR: /Users/runner/work/ic/ic/rs/nns/handlers/lifeline/impl/BUILD.bazel:35:16: CopyIdlFiles 
rs/nns/handlers/lifeline/impl/actor_idl_path failed: I/O exception during sandboxed execution: 
/private/tmp/zig-cache (No such file or directory)

.github/workflows/test-namespace-darwin.yaml doesn't actually use .github/actions/bazel which creates the /tmp/zig-cache directory. So let's create it.

It's unclear why this failure only happens sometimes and didn't happen on #4848 which re-introduced the hermetic CC toolchain.

This also drops:

          # Until we have a hermetic CC toolchain, tell bazel to use the "real" clang
          # (instead of Apple's, which sometimes breaks on wasm32)
          export CC=/opt/homebrew/opt/llvm/bin/clang

since we now have the hermetic CC toolchain available.