amazon-init.service: fix starting services at startup

We now make it happen later in the boot process so that multi-user
has already activated, so as to not run afoul of the logic in
switch-to-configuration.pl. It's not my favorite solution, but at
least it works. Also added a check to the VM test to catch the failure
so we don't break in future.

Fixes #23121

nixos/modules/virtualisation/amazon-init.nix

@@ -45,9 +45,8 @@ in {
     inherit script;
     description = "Reconfigure the system from EC2 userdata on startup";
 
-    wantedBy = [ "sshd.service" ];
-    before = [ "sshd.service" ];
-    after = [ "network-online.target" ];
+    wantedBy = [ "multi-user.target" ];
+    after = [ "multi-user.target" ];
     requires = [ "network-online.target" ];
  
     restartIfChanged = false;