electrum-ltc: only build on platforms.linux

darwin is broken

pkgs/applications/misc/electrum-ltc/default.nix

···
     '';
     homepage = https://electrum-ltc.org/;
     license = licenses.mit;
-    platforms = platforms.all;
+    platforms = platforms.linux;
     maintainers = with maintainers; [ asymmetric ];
   };
 }