Fix prepack script to use yarn instead of npm in package-scripts.yml

The build.prepack script in package-scripts.yml was using `npm run build`
which causes failures in CI. This is inconsistent with the rest of the
codebase which uses yarn.

Changed line 29 from:
  (npm run build >/dev/null 2>&1 || true)
To:
  (yarn run build >/dev/null 2>&1 || true)

This matches the fix previously made in packages/react-on-rails/package.json
(commit 9f254fb) but was missed in the root package-scripts.yml file.

Fixes CI error:
  Building react-on-rails seems to have failed!

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

Co-Authored-By: Claude <[email protected]>

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!'; }