feat: 👷 add Check formatting step to build job

Author: samjowen

.github/workflows/elixir.yml

@@ -29,6 +29,8 @@ jobs:
           restore-keys: ${{ runner.os }}-mix-
       - name: Install dependencies
         run: mix deps.get
+      - name: Check formatting
+        run: mix format --check-formatted
       - name: Run tests
         run: mix test