switch to programs.steam and proton-ge-bin from nixpkgs

proton-ge from nix-gaming is deprecated now that proton-ge-bin is in
nixpkgs

system/nix/substituters.nix

@@ -4,13 +4,11 @@
       # high priority since it's almost always used
       "https://cache.nixos.org?priority=10"
       "https://nix-community.cachix.org"
-      "https://nix-gaming.cachix.org"
     ];
 
     trusted-public-keys = [
       "cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY="
       "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
-      "nix-gaming.cachix.org-1:nbjlureqMbRAxR1gJ/f3hxemL9svXaZF/Ees8vCUUs4="
     ];
   };
 }

system/programs/steam.nix

@@ -0,0 +1,12 @@
+{pkgs, ...}: {
+  programs.steam = {
+    enable = true;
+    remotePlay.openFirewall = true;
+    localNetworkGameTransfers.openFirewall = true;
+    extraCompatPackages = with pkgs; [
+      proton-ge-bin
+    ];
+    extest.enable = true;
+    gamescopeSession.enable = true;
+  };
+}