[CI] Detect unpinned git submodules in GH PR checks

Author: chalin

.github/workflows/check-links.yml

@@ -11,6 +11,13 @@ jobs:
     steps:
       - uses: actions/checkout@v4
 
+      - name: Git submodules properly pinned?
+        run: |
+          npm run seq -- get:submodule pin:submodule
+          echo "If the diff check below fails, then update .gitmodules by pinning the named git"
+          echo "submodule(s); or undo the submodule update(s) if it happened by mistake."
+          npm run _diff:fail
+
       - name: Create NPM cache-hash input file
         run: |
           mkdir -p tmp