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/tests.yml

@@ -112,7 +112,7 @@ jobs:
         working-directory: src
         shell: ${{ matrix.config.shell }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
         with:
           fetch-depth: 0
           persist-credentials: false
@@ -139,11 +139,11 @@ jobs:
 
       - name: Set up QEMU
         if: matrix.config.base_image
-        uses: docker/setup-qemu-action@v3
+        uses: docker/setup-qemu-action@ce360397dd3f832beb865e1373c09c0e9f86d70a # v4.0.0
 
       - name: Set up Docker Buildx
         if: matrix.config.base_image
-        uses: docker/setup-buildx-action@v3
+        uses: docker/setup-buildx-action@4d04d5d9486b7bd6fa91e7baf45bbb4f8b9deedd # v4.0.0
 
       - name: Build Docker container
         if: matrix.config.base_image
@@ -161,7 +161,7 @@ jobs:
 
       - name: Setup msys and install required packages
         if: runner.os == 'Windows'
-        uses: msys2/setup-msys2@v2
+        uses: msys2/setup-msys2@e9898307ac31d1a803454791be09ab9973336e1c # v2.31.1
         with:
           msystem: ${{ matrix.config.msys_sys }}
           install: mingw-w64-${{ matrix.config.msys_env }} make git expect