CI: Automatically check changelog entries

.github/workflows/tests.yml

@@ -50,6 +50,14 @@ jobs:
         run: |
           go test ./...
 
+      - name: Check changelog files with calens
+        run: |
+          echo "install calens"
+          go install github.com/restic/calens@latest
+
+          echo "check changelog files"
+          calens
+
   lint:
     name: lint
     runs-on: ubuntu-latest