ci: add concurrency group to prevent duplicate runner builds

.gitea/workflows/android-build.yml

@@ -4,6 +4,12 @@ on:
   push:
   workflow_dispatch:
 
+# Only one run per branch at a time. A new push cancels any in-progress run,
+# preventing duplicate builds when the runner picks up the same event twice.
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 permissions:
   contents: write   # required for creating releases
   packages: write