flatpak: Set override for Steam to have access to games directory

Signed-off-by: George Kaklamanos <gkaklas@gkaklas.gr>

home-manager/flatpak.nix

@@ -42,5 +42,10 @@
   ];
   services.flatpak.overrides = {
     global.Context.filesystems = ["xdg-config/gtk-3.0"];
+    "com.valvesoftware.Steam".Context = {
+      filesystems = [
+        "/mnt/games:rw"
+      ];
+    };
   };
 }