Add nix-deck kernel to CI cache

The Steam Deck (nix-deck) kernel from Jovian-NixOS is expensive to
build. Pre-building it in CI saves significant time on updates.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.nix

@@ -268,7 +268,10 @@
           src = inputs.gastown;
           version = "unstable-${gastownRev}";
         };
-      }
+      } // (if system == "x86_64-linux" then {
+        # nix-deck kernel from Jovian-NixOS (Steam Deck) - expensive to build
+        "nix-deck-kernel" = self.nixosConfigurations.nix-deck.config.boot.kernelPackages.kernel;
+      } else {})
     );
 
     # Flake apps