fix: download app-builder binary explicitly after --ignore-scripts install

--ignore-scripts skipped app-builder-bin postinstall which downloads the
electron-builder binary, causing ENOENT at build time.
Add explicit step: node node_modules/app-builder-bin/install.js

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/build-linux.yml

@@ -29,6 +29,9 @@ jobs:
       - name: Install root workspace dependencies
         run: npm install --ignore-scripts --legacy-peer-deps
 
+      - name: Download app-builder binary
+        run: node node_modules/app-builder-bin/install.js
+
       - name: Vite build
         run: npx electron-vite build
         working-directory: apps/desktop