rename notificationFd to notification-fd

it's perfectly valid syntax and consistent with the s6 spelling

pkgs/liminix-tools/services/default.nix

@@ -28,11 +28,10 @@ let
   } @ args: stdenvNoCC.mkDerivation {
     # we use stdenvNoCC to avoid generating derivations with names
     # like foo.service-mips-linux-musl
-    inherit name serviceType up down run;
+    inherit name serviceType up down run notification-fd;
     buildInputs = dependencies ++ contents;
     dependencies = builtins.map (d: d.name) dependencies;
     contents = builtins.map (d: d.name) contents;
-    notificationFd = notification-fd;
     builder = ./builder.sh;
   };