ci: specify phpunit version

Author: Greg Bowler

.github/workflows/ci.yml

@@ -58,6 +58,7 @@ jobs:
         env:
           XDEBUG_MODE: cover
         with:
+          version: "10.1"
           php_version: ${{ matrix.php }}
           php_extensions: xdebug
           coverage_text: _coverage/coverage.txt