Add quote for OS list

Author: pH-7

.github/workflows/test-ci.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ${{ matrix.os }}
     strategy:
       matrix:
-        os: [ ubuntu-latest ]
+        os: [ 'ubuntu-latest' ]
         php-versions: [ '7.3', '7.4', '8.0', '8.1' ]
         phpunit-version: [ '9.5' ]
     name: PHP ${{ matrix.php-versions }} Test on ${{ matrix.os }}