bugfix: be careful about GITHUB_BASE_REF in spack style

`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.

share/spack/qa/run-style-tests

@@ -16,8 +16,13 @@
 #
 . "$(dirname $0)/setup.sh"
 
+BASE=""
+if [ -n "$GITHUB_BASE_REF" ]; then
+    BASE="--base ${GITHUB_BASE_REF}"
+fi
+
 # verify that the code style is correct
-spack style --root-relative
+spack style --root-relative $BASE
 
 # verify that the license headers are present
 spack license verify