make genepi a remote builder

hosts/genepi/builder.nix

@@ -0,0 +1,18 @@
+{ 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 ];
+  };
+
+  users.groups."${username}" = { };
+
+  nix.settings.trusted-users = [ username ];
+}