Add concurrency check (space-ros#225)

Author: dsobek

.github/workflows/earthly-build.yaml

@@ -8,6 +8,11 @@ on:
   schedule:
     - cron: "0 11 * * *"
 
+# Ensures that only one workflow per branch will run at a time
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref_name }}
+  cancel-in-progress: true
+
 env:
   VCS_REF: ${{ github.sha }}
   GHCR_IMAGE_NAME: "ghcr.io/space-ros/space-ros"