[ci] re-enable mercure tests on linux

Author: jrushlow

.github/workflows/ci-linux.yaml

@@ -11,7 +11,7 @@ on:
 env:
     PHPUNIT_FLAGS: "-v"
     SYMFONY_PHPUNIT_DIR: "$HOME/symfony-bridge/.phpunit"
-    MAKER_SKIP_MERCURE_TEST: 1
+    MAKER_SKIP_MERCURE_TEST: false
 
 jobs:
     coding-standards: