adding php8.1 as an experimental build (#514)

a failing experimental job no longer stops the whole build, although we will see a red X. There is a feature request to be able to set jobs as "allowed-to-fail", but until that feature exists this looks like the best github actions can do.

.github/workflows/php.yml

@@ -10,19 +10,26 @@ on:
 jobs:
   build:
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
+    continue-on-error: ${{ matrix.experimental }}
     strategy:
+      fail-fast: false
       matrix:
-        operating-system: [ubuntu-latest]
-        php-versions: ['7.4', '8.0']
+        php-version: ['7.4', '8.0']
+        os: [ubuntu-latest]
+        experimental: [false]
+        include:
+          - php-version: 8.1
+            os: ubuntu-latest
+            experimental: true
 
     steps:
     - uses: actions/checkout@v2
 
     - name: Setup PHP
       uses: shivammathur/setup-php@v2
       with:
-        php-version: ${{ matrix.php-versions }}
+        php-version: ${{ matrix.php-version }}
         coverage: xdebug
         tools: php-cs-fixer
         extensions: ast, grpc