nixos/acme: Fix allowKeysForGroup not applying immediately

Previously setting `allowKeysForGroup = true; group = "foo"` would not
apply the group permission change of the certificates until the service
gets restarted. This commit fixes this by making systemd restart the
service every time it changes.

Note that applying this commit to a system with an already running acme
systemd service doesn't fix this immediately and you still need to wait
for the next refresh (or call `systemctl restart acme-<domain>`). Once
everybody's service has restarted once this should be a problem of the
past.