CI: Add step to purge Cloudflare cache

This was previously run as a 'post-deploy-actions.sh' script on the
server via SSH.

.github/workflows/main.yml

@@ -32,3 +32,8 @@ jobs:
         projectName: start-ubuntu-mate-org
         directory: _site
         gitHubToken: ${{ secrets.GITHUB_TOKEN }}
+
+    - name: Purge Cloudflare Cache
+      if: ${{ github.event_name == 'push' && github.repository == 'ubuntu-mate/start.ubuntu-mate.org' && github.ref == 'refs/heads/master' }}
+      run: |
+        curl -X POST --fail-with-body https://api.cloudflare.com/client/v4/zones/${{secrets.CLOUDFLARE_ZONE_ID}}/purge_cache -H "Authorization: Bearer ${{secrets.CLOUDFLARE_API_TOKEN}}" -H "Content-Type: application/json" --data '{"purge_everything":true}'