Manually install Heroku CLI.

As recommended in AkhileshNS/heroku-deploy#188.

Author: Zimmi48

.github/workflows/deploy_heroku.yml

@@ -12,6 +12,8 @@ jobs:
       - uses: actions/checkout@v4
       - name: Build release Docker image
         run: docker build -f release.Dockerfile . -t coqbot
+      - name: Install Heroku CLI
+        run: curl https://cli-assets.heroku.com/install.sh | sh
       - uses: akhileshns/[email protected]
         with:
           heroku_api_key: ${{secrets.HEROKU_API_KEY}}