nixos/sshd: Fix socket activated SSH connections entering failed state.

In afeb76d628, sshd.service and
sshd@.service were switched to Type=notify. This apparently works for
sshd.service, but not for sshd@.service. Given that the reason for
this working with sshd.service isn't exactly clear, let's revert it
for both of them for now, and revisit Type=notify later.

nixos/modules/services/networking/ssh/sshd.nix

@@ -593,7 +593,6 @@ in
           environment.LD_LIBRARY_PATH = nssModulesPath;
 
           serviceConfig = {
-            Type = "notify";
             ExecStart = lib.concatStringsSep " " [
               "-${lib.getExe' cfg.package "sshd"}"
               "-i"
@@ -618,7 +617,6 @@ in
           restartTriggers = [ config.environment.etc."ssh/sshd_config".source ];
 
           serviceConfig = {
-            Type = "notify";
             Restart = "always";
             ExecStart = lib.concatStringsSep " " [
               (lib.getExe' cfg.package "sshd")

nixos/tests/openssh.nix

@@ -274,5 +274,8 @@ in {
             "ssh -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no -i privkey.snakeoil server-no-pam true",
             timeout=30
         )
+
+    # None of the per-connection units should have failed.
+    server_lazy.fail("systemctl is-failed 'sshd@*.service'")
   '';
 })