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>

.gitea/workflows/ci.yml

@@ -51,6 +51,7 @@ jobs:
             custom-beads
             custom-gastown
             qt-pinned-jellyfin-media-player
+            nix-deck-kernel
           )
 
           FAILED=()