Give gamescope cap_sys_nice so it can renice itself

Signed-off-by: magic_rb <magic_rb@redalder.org>

nixos/common/steam.nix

@@ -1,9 +1,12 @@
 { pkgs, lib, ... }:
 {
   programs.gamemode.enable = true;
+  programs.gamescope = {
+    enable = true;
+    capSysNice = true;
+  };
 
   environment.systemPackages = with pkgs; [
-    gamescope
     mangohud
   ];