ci: fix Heroku CLI installation in GitHub Actions workflow

- Add explicit Heroku CLI installation step before login
- Update container registry login and push commands
- Configure environment variables for Heroku app

Author: CynthiaWahome

.github/workflows/cd.yml

@@ -11,6 +11,10 @@ jobs:
     steps:
       - uses: actions/checkout@v2
 
+      - name: Install Heroku CLI
+        run: |
+          curl https://cli-assets.heroku.com/install.sh | sh
+      
       - name: Login to Heroku Container Registry
         env:
           HEROKU_API_KEY: ${{ secrets.HEROKU_API_KEY }}