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.

nixos/modules/services/monitoring/prometheus/alertmanager.nix

@@ -155,7 +155,7 @@ in {
 
       systemd.services.alertmanager = {
         wantedBy = [ "multi-user.target" ];
-        after    = [ "network.target" ];
+        after    = [ "network-online.target" ];
         preStart = ''
            ${lib.getBin pkgs.envsubst}/bin/envsubst -o "/tmp/alert-manager-substituted.yaml" \
                                                     -i "${alertmanagerYml}"