build: allow installing without git

[branchvincent]

This allows installing from a tarball rather than requiring a git clone, which is helpful for downstream distributors (such as Homebrew).

Before:

$ pip3 install https://github.com/mgbellemare/Arcade-Learning-Environment/archive/refs/tags/v0.8.1.tar.gz
...
× Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [25 lines of output]
      fatal: not a git repository (or any of the parent directories): .git

After:

$ pip3 install https://github.com/branchvincent/Arcade-Learning-Environment/archive/refs/heads/tarball.tar.gz
...
Successfully built ale-py