cryptomator: fail more gracefully on darwin

It was blocking channels since PR #241154 via:
https://hydra.nixos.org/build/231587712/nixlog/1/tail

pkgs/tools/security/cryptomator/default.nix

@@ -1,4 +1,4 @@
-{ lib, fetchFromGitHub
+{ lib, stdenv, fetchFromGitHub
 , autoPatchelfHook
 , fuse3
 , maven, jdk, makeShellWrapper, glib, wrapGAppsHook
@@ -10,6 +10,7 @@ let
     jdk = jdk;
   };
 in
+assert stdenv.isLinux; # better than `called with unexpected argument 'enableJavaFX'`
 mavenJdk.buildMavenPackage rec {
   pname = "cryptomator";
   version = "1.9.1";