ci: update caddy job to use GitHub Releases

The caddyserver.com download page is unreliable, and it also recommends to use GitHub releases instead.

Author: Ayesh

.github/actions/setup-caddy/action.yml

@@ -5,6 +5,6 @@ runs:
     - shell: bash
       run: |
         set -x
-        sudo curl 'https://caddyserver.com/api/download?os=linux&arch=amd64' -o /usr/bin/caddy
+        curl -s -L --max-time 120 --retry-delay 10 --retry 3 'https://github.com/caddyserver/caddy/releases/download/v2.7.6/caddy_2.7.6_linux_amd64.tar.gz' | sudo tar -xz -C /usr/bin caddy
         sudo chmod +x /usr/bin/caddy
         sudo caddy start --config ext/curl/tests/Caddyfile