ci: explicitly fetch PR base SHA before diff

.gitea/workflows/validate-pr.yml

@@ -19,6 +19,10 @@ jobs:
         run: |
           set -e
           base="${{ github.event.pull_request.base.sha }}"
+          # act's actions/checkout@v4 doesn't reliably pull the base SHA
+          # into the local history even with fetch-depth: 0. Fetch it
+          # explicitly so the three-dot diff resolves.
+          git fetch --depth=1 origin "$base" || git fetch --depth=1 origin
           changed=$(git diff --name-only "$base"...HEAD -- 'recipes/' \
             | awk -F/ '{print $2}' | sort -u)
           if [[ -z "$changed" ]]; then