move nix remote builder to a module

hosts/genepi/builder.nix

@@ -1,18 +1,7 @@
 { keys, ... }:
-let
-  username = "nixremote";
-in
 {
-  users.users."${username}" = {
-    createHome = true;
-    home = "/home/${username}";
-    isSystemUser = true;
-    group = username;
-    useDefaultShell = true;
-    openssh.authorizedKeys.keys = [ keys.hosts.haze ];
+  roles.remote-builder = {
+    enable = true;
+    authorizedKeys = [ keys.hosts.haze ];
   };
-
-  users.groups."${username}" = { };
-
-  nix.settings.trusted-users = [ username ];
 }