Add qt-pinned-stremio to CI cache

Stremio uses qt5webengine, so it benefits from the separate
nixpkgs-qt input to avoid rebuild churn.

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
+            qt-pinned-stremio
             nix-deck-kernel
           )