ci: fetch necessary refs

Follow-up to #4785. Whilst comparing against the actual PR base is correct, we need to fetch the ref first before `git` can resolve it.

Pull-Request: #4786.

.github/workflows/ci.yml

@@ -82,6 +82,7 @@ jobs:
           !contains(github.event.pull_request.labels.*.name, 'internal-change')
         run: |
           git fetch origin master:master
+          git fetch origin ${{ github.event.pull_request.base.ref }}:${{ github.event.pull_request.base.ref }}
           ./scripts/ensure-version-bump-and-changelog.sh
         env:
           HEAD_SHA: ${{ github.event.pull_request.head.sha }}