Prevent unlocked dependencies in CI (#383)

Author: maennchen

.github/workflows/main.yml

@@ -35,7 +35,7 @@ jobs:
           elixir-version: ${{ matrix.elixir }}
 
       - name: Install dependencies
-        run: mix deps.get
+        run: mix deps.get --check-locked
 
       - name: Check no unused dependencies
         run: mix deps.unlock --check-unused