duti: set kernel to darwin18

Unfortunately, we can’t do arm64 multi-arch with the current nixpkgs
sdk, which duti assumes if your kernel is darwin20 (Big Sur). To fix
this, we can just hardcode to darwin18 which should be compatible with
Nixpkgs Darwin.

Fixes #105751

pkgs/os-specific/darwin/duti/default.nix

@@ -12,7 +12,14 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [autoreconfHook];
   buildInputs = [darwin.apple_sdk.frameworks.ApplicationServices];
-  configureFlags = ["--with-macosx-sdk=/homeless-shelter"];
+  configureFlags = [
+    "--with-macosx-sdk=/homeless-shelter"
+
+    # needed to prevent duti from trying to guess our sdk
+    # NOTE: this is different than stdenv.hostPlatform.config!
+    "--host=x86_64-apple-darwin18"
+  ];
+
   meta = with lib; {
     description = "A command-line tool to select default applications for document types and URL schemes on Mac OS X";
     longDescription = ''