`spack style` previously used a Travis CI variable to figure out
what the base branch of a PR was, and this was apparently also set
on `develop`. We switched to `GITHUB_BASE_REF` to support GitHub
Actions, but it looks like this is set to `""` in pushes to develop,
so `spack style` breaks there.
This PR does two things:
- [x] Remove `GITHUB_BASE_REF` knowledge from `spack style` entirely
- [x] Handle `GITHUB_BASE_REF` in style scripts instead, and explicitly
pass the base ref if it is present, but don't otherwise.
This makes `spack style` *not* dependent on the environment and fixes
handling of the base branch in the right place.
6104c31556