Merge pull request #193009 from SuperSandro2000/nix-daemon-localhost

nixos/modules/services/misc/nix-daemon.nix

···
               '';
             };
             protocol = mkOption {
-              type = types.enum [ "ssh" "ssh-ng" ];
+              type = types.enum [ null "ssh" "ssh-ng" ];
               default = "ssh";
               example = "ssh-ng";
               description = lib.mdDoc ''
                 The protocol used for communicating with the build machine.
                 Use `ssh-ng` if your remote builder and your
                 local Nix version support that improved protocol.
+
+                Use `null` when trying to change the special localhost builder
+                without a protocol which is for example used by hydra.
               '';
             };
             system = mkOption {
···
         concatMapStrings
           (machine:
             (concatStringsSep " " ([
-              "${machine.protocol}://${optionalString (machine.sshUser != null) "${machine.sshUser}@"}${machine.hostName}"
+              "${optionalString (machine.protocol != null) "${machine.protocol}://"}${optionalString (machine.sshUser != null) "${machine.sshUser}@"}${machine.hostName}"
               (if machine.system != null then machine.system else if machine.systems != [ ] then concatStringsSep "," machine.systems else "-")
               (if machine.sshKey != null then machine.sshKey else "-")
               (toString machine.maxJobs)