ci: pin root workflow actions to immutable SHAs

Pin every external action used by the root GitHub workflow set to a full-length
commit SHA and keep the selected release tag as a same-line comment.

Update the root workflows to the latest stable tagged action versions found
during the audit while preserving existing release guards, permissions, and
reusable workflow wiring.

closes https://github.com/official-stockfish/Stockfish/pull/6754

No functional change.

.github/workflows/games.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - name: Checkout SF repo 
-        uses: actions/checkout@v4
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           ref: ${{ github.event.pull_request.head.sha }}
           path: Stockfish
@@ -20,7 +20,7 @@ jobs:
         run: make -j build debug=yes
 
       - name: Checkout fastchess repo
-        uses: actions/checkout@v4
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           repository: Disservin/fastchess
           path: fastchess