Restructure flake.nix for a simpler config for building different images off the same NixOS config.

nix/configuration/roles/steam_run_free/default.nix

@@ -17,13 +17,9 @@
     };
   };
 
-  config = lib.mkIf config.me.steam_run_free.enable (
-    lib.mkMerge [
-      {
-        environment.systemPackages = with pkgs; [
-          steam-run-free
-        ];
-      }
-    ]
-  );
+  config = lib.mkIf config.me.steam_run_free.enable {
+    environment.systemPackages = with pkgs; [
+      steam-run-free
+    ];
+  };
 }