ci: enable mtail for CI

github deployments: be lenient on the whitespace in sed replace of
mtail_address

.github/workflows/test-and-deploy-ipv4only.yaml

@@ -74,6 +74,7 @@ jobs:
       - run: |
           cmdeploy init staging-ipv4.testrun.org
           sed -i 's#disable_ipv6 = False#disable_ipv6 = True#' chatmail.ini
+          sed -i 's/#\s*mtail_address/mtail_address/' chatmail.ini
 
       - run: cmdeploy run --verbose --skip-dns-check