Fix phpunit version used in s3 external testing

Signed-off-by: Joas Schilling <coding@schilljs.com>

.github/workflows/s3-external.yml

@@ -44,7 +44,7 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php-versions }}
-          tools: phpunit
+          tools: phpunit:8.5.2
           extensions: mbstring, fileinfo, intl, sqlite, pdo_sqlite, zip, gd
 
       - name: Set up Nextcloud
@@ -93,7 +93,7 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php-versions }}
-          tools: phpunit
+          tools: phpunit:8.5.2
           extensions: mbstring, fileinfo, intl, sqlite, pdo_sqlite, zip, gd
 
       - name: Set up Nextcloud