playground workflow: fix stack name

.github/workflows/deploy-playground.yml

@@ -29,7 +29,7 @@ jobs:
     name: Create an EC2 instance and configure docker-compose stack
     runs-on: ubuntu-latest
     env:
-      STACK_NAME: joystream-playground-{{ github.event.inputs.branchName }}-${{ github.run_number }}
+      STACK_NAME: joystream-playground-${{ github.event.inputs.branchName }}-${{ github.run_number }}
     steps:
       - name: Checkout
         uses: actions/checkout@v2