Fix prepack script to use yarn instead of npm

Change the build.prepack script to use 'yarn run build' instead of
'npm run build'. In a yarn workspace, using npm to run scripts that
depend on workspace features (like 'yarn workspace react-on-rails run build')
causes build failures.

This fixes the yalc package publishing failures in CI where the prepack
script would fail silently, resulting in packages being published without
the built files, leading to downstream errors.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: justin808

package-scripts.yml

@@ -26,7 +26,7 @@ scripts:
       # 4. If it failed, print an error message (still follow https://docs.npmjs.com/cli/v8/using-npm/scripts#best-practices).
       script: >
         [ -f packages/react-on-rails/lib/ReactOnRails.full.js ] ||
-          (npm run build >/dev/null 2>&1 || true) &&
+          (yarn run build >/dev/null 2>&1 || true) &&
           [ -f packages/react-on-rails/lib/ReactOnRails.full.js ] ||
           { echo 'Building react-on-rails seems to have failed!'; }