Fix spurious $$ in workflow.

Author: stephankramer

.github/workflows/docker_firedrake-parmmg_main.yml

@@ -30,7 +30,7 @@ permissions: {}
 
 # Stop the workflow if a new run is requested
 concurrency:
-  group: $${{ github.workflow }}-${{ github.ref }}
+  group: ${{ github.workflow }}-${{ github.ref }}
   cancel-in-progress: true
 
 jobs: