Force Composer v1 to be installed for tests

Until such a time that `wikimedia/composer-merge-plugin` supports Composer 2.

Refs: https://github.com/octobercms/october/pull/5330#issuecomment-715988820

.github/workflows/tests.yml

@@ -62,7 +62,7 @@ jobs:
         uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.phpVersion }}
-          tools: composer
+          tools: composer:1.10.16
           extensions: ${{ env.extensions }}
 
       - name: Setup dependency cache