ci: cache extensions (#223)

Try caching the PHP extensions since they take a long time to
build. We try this both in the lint job and test job.

We also temporarily reduce the number of versions under test in
order to increase velocity.

.github/workflows/on-pull-request.yaml

@@ -9,6 +9,9 @@ on:
 jobs:
   lint:
     runs-on: ubuntu-24.04
+    env:
+      php-version: 8.0
+      extensions: grpc, protobuf
 
     steps:
       - uses: actions/checkout@v4
@@ -29,11 +32,30 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
 
+      - name: Set cache key
+        id: cache-key
+        run: echo "key=php-${{ env.php-version }}" >> $GITHUB_OUTPUT
+
+      - name: Setup cache environment
+        id: extcache
+        uses: shivammathur/cache-extensions@v1
+        with:
+          php-version: ${{ env.php-version }}
+          extensions: ${{ env.extensions }}
+          key: ${{ steps.cache-key.outputs.key }}
+
+      - name: Cache extensions
+        uses: actions/cache@v4
+        with:
+          path: ${{ steps.extcache.outputs.dir }}
+          key: ${{ steps.extcache.outputs.key }}
+          restore-keys: ${{ steps.extcache.outputs.key }}
+
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '8.0'
-          extensions: grpc, protobuf
+          php-version: ${{ env.php-version }}
+          extensions: ${{ env.extensions }}
           tools: composer
 
       - name: Install dependencies
@@ -47,20 +69,41 @@ jobs:
       max-parallel: 1
       fail-fast: true
       matrix:
-        php-version: ['7.4', '8.0', '8.1', '8.2', '8.3', '8.4']
+        php-version: ['7.4', '8.4']
+        #php-version: ['7.4', '8.0', '8.1', '8.2', '8.3', '8.4'] # TODO: Uncomment to test all PHP versions
     runs-on: ubuntu-24.04
 
     env:
       MOMENTO_API_KEY: ${{ secrets.ALPHA_TEST_AUTH_TOKEN }}
+      extensions: grpc, protobuf
 
     steps:
       - uses: actions/checkout@v4
 
+      - name: Set cache key
+        id: cache-key
+        run: echo "key=php-${{ matrix.php-version }}" >> $GITHUB_OUTPUT
+
+      - name: Setup cache environment
+        id: extcache
+        uses: shivammathur/cache-extensions@v1
+        with:
+          php-version: ${{ matrix.php-version }}
+          extensions: ${{ env.extensions }}
+          key: ${{ steps.cache-key.outputs.key }}
+
+      - name: Cache extensions
+        uses: actions/cache@v4
+        with:
+          path: ${{ steps.extcache.outputs.dir }}
+          key: ${{ steps.extcache.outputs.key }}
+          restore-keys: ${{ steps.extcache.outputs.key }}
+
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php-version }}
-          extensions: grpc, protobuf
+          extensions: ${{ env.extensions }}
           tools: composer
 
       - name: Install dependencies