Modify `test-expand.sh` so that `test` binary is generated as a tmp file

Currently, `.github/workflows/test-expand.sh` runs `rustc` that creates a binary named `test` in the current working directory.  This is a bit troublesome when testing locally because it creates an untracked binary file in the repository.  Woo, dangerous.  Let's set a option so that `rustc` generates a binary in the `/tmp` directory instead!