ci: run style unit tests only if we target develop (#27472)

Some tests assume the base branch is develop, but this branch may not
have been checked out.

.github/workflows/setup_git.sh

@@ -1,9 +1,8 @@
 #!/usr/bin/env sh
 git config --global user.email "spack@example.com"
 git config --global user.name "Test User"
-# With fetch-depth: 0 we have a remote develop
-# but not a local branch. Don't do this on develop
-if [ "$(git branch --show-current)" != "develop" ]
-then
-  git branch develop origin/develop
+
+# create a local pr base branch
+if [[ -n $GITHUB_BASE_REF ]]; then
+    git fetch origin "${GITHUB_BASE_REF}:${GITHUB_BASE_REF}"
 fi