We don't need to check out php-src for 7.x

The following check needs 8.x, so also depend on 8.x for the
download. Perhaps this could also be turned into a discrete step...

Author: NattyNarwhal

.github/workflows/ci.yml

@@ -32,6 +32,7 @@ jobs:
         with:
           php-version: ${{matrix.version}}
       - name: Checkout "php/php-src"
+        if: "${{ matrix.version >= '8.0' }}"
         uses: actions/checkout@v3
         with:
           repository: php/php-src