nixos/nix: add option to preface remote machines with a comment

I recently ran into an issue where I wasn't able to use the DNS hostname of a
machine inside the /etc/nix/machines file. Nothing to do with NixOS or Nix
itself, just a weird networking setup I was in. This meant I had to use the
plain IPv4 and IPv6 addresses of those machines in these files.

I still like to have some reference to what these machines are, so I've always
put a comment above the remote machine line with the DNS name of the machine,
and then use the hostName option for the IP address.

This adds the nix.buildMachines.*.comment option. Allows us to do the following.

    {
      nix.buildMachines = [
        {
          protocol = "ssh-ng";
          sshUser = "nixbld";
          hostName = "192.168.0.222";
          comment = "Some very powerful machine in Belgium";
        }
      ];
    }

Which would result in the following /etc/nix/machines entry.

    # Some very powerful machine in Belgium
    ssh-ng://[email protected] - - 1 1 - - -

nixos/modules/config/nix-remote-build.nix

@@ -31,7 +31,7 @@ let
 
   buildMachinesText = concatMapStrings (
     machine:
-    (concatStringsSep " " (
+    ((optionalString (machine.comment != null) "# ${machine.comment}\n") + concatStringsSep " " (
       [
         "${optionalString (machine.protocol != null) "${machine.protocol}://"}${
           optionalString (machine.sshUser != null) "${machine.sshUser}@"
@@ -201,6 +201,15 @@ in
                   If null, SSH will use its regular known-hosts file when connecting.
                 '';
               };
+              comment = mkOption {
+                type = types.nullOr types.str;
+                default = null;
+                example = "This is a powerful builder located in Belgium";
+                description = ''
+                  A comment to describe the builder. Could be useful for noting the full
+                  DNS name of a machine, and using an IP address as hostName.
+                '';
+              };
             };
           }
         );