fix(ci): fix Tauri PR check build - remove invalid --bundles none flag

--bundles none is not a valid value for macOS Tauri builds. Remove TAURI_SIGNING_PRIVATE_KEY empty string env var (leaving it unset tells Tauri to skip signing) and revert to plain debug build command. If updater artifacts are still created, use --bundles app to skip DMG/updater bundle.

---
Developed by Claude Sonnet 4.6

Author: houke

.github/workflows/fluxby-pr-check.yml

@@ -122,4 +122,4 @@ jobs:
 
       - name: Build Tauri app (debug)
         working-directory: apps/tauri
-        run: npx tauri build --debug --bundles none
+        run: npx tauri build --debug --bundles app