chore: pin GitHub Actions to full commit SHAs

Pins all GitHub Actions uses: references to full commit SHAs
to prevent supply chain attacks via mutable action tags.
Original tag references are kept as inline comments.

.github/workflows/tests.yml

@@ -18,10 +18,10 @@ jobs:
     name: PHP ${{ matrix.php }}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
 
       - name: Setup PHP
-        uses: shivammathur/setup-php@v2
+        uses: shivammathur/setup-php@7c071dfe9dc99bdf297fa79cb49ea005b9fcadbc # v2
         with:
           php-version: ${{ matrix.php }}
           extensions: dom, curl, libxml, mbstring, zip