Add vendor cache

Author: patrikjuvonen

.github/workflows/php.yml

@@ -15,6 +15,10 @@ jobs:
     name: Build PHP version ${{ matrix.php-versions }}
     steps:
     - uses: actions/checkout@v2
+    - uses: actions/cache@v2
+      with:
+        path: vendor
+        key: ${{ runner.os }}-${{ hashFiles('**/composer.lock') }}
     - uses: shivammathur/setup-php@v2
       with:
         php-version: ${{ matrix.php-versions }}