Add nightly PHP 8.1 dev to github actions (#1763)

Author: Mark Baker

.github/workflows/main.yml

@@ -10,6 +10,7 @@ jobs:
           - '7.3'
           - '7.4'
           - '8.0'
+          - '8.1'
 
     name: PHP ${{ matrix.php-version }}
 
@@ -37,7 +38,7 @@ jobs:
 
       - name: Delete composer lock file
         id: composer-lock
-        if: ${{ matrix.php-version == '8.0' }}
+        if: ${{ matrix.php-version == '8.0' || matrix.php-version == '8.1' }}
         run: |
           rm composer.lock
           echo "::set-output name=flags::--ignore-platform-reqs"