systems/git: make sshd startup less racy and flaky

hosts/git/default.nix

@@ -30,6 +30,11 @@ in
   services.openssh.listenAddresses = [{
     addr = "[${ipv6.openssh}]";
   }];
+  # Defaults to network.target, but networkd may take a while to settle and set up
+  # the required (additional) IPv6 address, leading to sshd to not being able to
+  # bind to the requested IP, crashing 5 times and running into the default
+  # restart counter limit (5).
+  systemd.services.sshd.after = [ "network-online.target" ];
 
   bagel.services.forgejo = {
     enable = true;