workflows: rename from yaml to yml

Rename build and test workflow to yml instead of yaml.
Also, test workflow needs permission to download artifacts

Author: DematteisGiacomo

.github/workflows/build-and-target-test.yaml .github/workflows/build-and-target-test.yml

@@ -49,6 +49,9 @@ jobs:
     runs-on: ${{ matrix.device }}
     name: Target Test - ${{ matrix.device }}
 
+    permissions:
+      contents: write
+
     container:
       image: ghcr.io/hello-nrfcloud/firmware:docker-v1.0.3
       options: --privileged