CI: allow only one build to take place at a time

This will prevent the CI from building the same packages twice when a
build gets triggered while there are other ongoing builds.

.github/workflows/CI.yml

@@ -10,6 +10,8 @@ on:
 jobs:
   CI:
     runs-on: ubuntu-latest
+    concurrency:
+      group: CI
     steps:
     - name: Checkout
       uses: actions/checkout@v3