nixos/systemd: fix modprobe (#507397)

nixos/modules/system/boot/systemd.nix

@@ -792,6 +792,10 @@ in
       path = [ pkgs.util-linux ];
       overrideStrategy = "asDropin";
     };
+    systemd.services."modprobe@" = {
+      restartIfChanged = false;
+      serviceConfig.ExecSearchPath = lib.makeBinPath [ pkgs.kmod ];
+    };
     systemd.services.systemd-random-seed.restartIfChanged = false;
     systemd.services.systemd-remount-fs.restartIfChanged = false;
     systemd.services.systemd-update-utmp.restartIfChanged = false;

nixos/tests/systemd-misc.nix

@@ -60,5 +60,10 @@ in
     machine.succeed("systemctl status example.service | grep 'Active: active'")
 
     machine.succeed("systemctl show --property TasksMax --value user-1000.slice | grep 100")
+
+    with subtest("modprobe@ services work"):
+      modprobe_service_status = machine.succeed("systemctl show --property ExecMainStatus modprobe@configfs.service")
+      print(modprobe_service_status)
+      t.assertEqual("ExecMainStatus=0\n", modprobe_service_status)
   '';
 }