[CI] Improve fetching of changed files

PRs always use a merge into the repo on checkout, so checking for
`HEAD^1` will show all the changes regardless of the number of commits
in a PR

.github/workflows/static_checks.yml

@@ -32,7 +32,7 @@ jobs:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         run: |
           if [ "${{ github.event_name }}" == "pull_request" ]; then
-            files=$(git diff-tree --no-commit-id --name-only -r HEAD~${{ github.event.pull_request.commits }}..HEAD 2> /dev/null || true)
+            files=$(git diff-tree --no-commit-id --name-only -r HEAD^1..HEAD 2> /dev/null || true)
           elif [ "${{ github.event_name }}" == "push" -a "${{ github.event.forced }}" == "false" -a "${{ github.event.created }}" == "false" ]; then
             files=$(git diff-tree --no-commit-id --name-only -r ${{ github.event.before }}..${{ github.event.after }} 2> /dev/null || true)
           fi