CI/frontend: Add missing install flag for prod flavor

Also restrict linting, which requires dev packages, to the `dev`
flavor.
This makes the frontend build setup indentical to the backend.

.github/workflows/ci.yml

@@ -67,7 +67,7 @@ jobs:
           registry-url: 'https://registry.npmjs.org'
 
       - name: Install (Prod dependencies only)
-        run: npm install
+        run: npm install --prod
         if: ${{ matrix.flavor == 'prod'}}
         working-directory: ${{ matrix.flavor }}/frontend
 
@@ -76,7 +76,8 @@ jobs:
         run: npm install
         working-directory: ${{ matrix.flavor }}/frontend
 
-      - name: Lint 
+      - name: Lint
+        if: ${{ matrix.flavor == 'dev'}}
         run: npm run lint
         working-directory: ${{ matrix.flavor }}/frontend