nixos/alertmanager: start after the network-online target

If the host network stack is slow to start, the alertmanager fails to
start with this error message:

    caller=main.go:256 msg="unable to initialize gossip mesh" err="create memberlist: Failed to get final advertise address: No private IP address found, and explicit IP not provided"

This bug can be reproduced by shutting down the network stack and
restarting the alertmanager.

Note I don't know why I didn't hit this issue with previous
alertmanager releases.