CI: Printing echobot invite link after cmdeploy run sometimes fails

[missytake]

• Server OS (Operating System) - preferably Debian 12: Debian 12
• On which OS you run cmdeploy: github CI
• chatmail/relay version: https://github.com/chatmail/relay/actions/runs/17808076868/job/50624784672?pr=639

Since #620 we print the echobot link at the end of CI. It seems that sometimes echobot.service hasn't started yet at this point, and /var/lib/echobot/invite-link.txt doesn't exist yet. This makes the CI flaky.

[missytake]

Until postfix + dovecot are enabled, echobot fails and tries to restart every 30 seconds. If cmdeploy run takes less than 30 seconds between starting postfix + dovecot and finishing, there is the possibility that echobot hasn't started yet, and the file doesn't exist. Then CI fails.