Update firebase-hosting-pull-request.yml

Author: mariush2

.github/workflows/firebase-hosting-pull-request.yml

@@ -8,6 +8,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - run: npm install
       - run: npm run build
       - uses: FirebaseExtended/action-hosting-deploy@v0
         with: