enabled gamescope

capSysNice allows game scope to set nice value for scheduler

: References
https://man7.org/linux/man-pages/man7/capabilities.7.html
CAP_SYS_NICE

https://wiki.archlinux.org/title/Gamescope
3.3 Setting Gamescopes priority

common/optional/steam.nix

@@ -13,5 +13,9 @@
       extraCompatPackages = with pkgs; [proton-ge-bin];
       extest.enable = true;
     };
+    gamescope = {
+      enable = true;
+      capSysNice = true;
+    };
   };
 }