fix(CI): Initialize non-existent bundler manifests (#75987)
These jobs were failing because they first attempt to open the existing file to compare, and there was no pre-existing file:
https://github.com/vercel/next.js/actions/workflows/rspack-update-tests-manifest.yml
Fixed this by running:
```
echo '{}' > /home/bgw.linux/next.js/test/rspack-dev-tests-manifest.json
echo '{}' > /home/bgw.linux/next.js/test/rspack-build-tests-manifest.json
node test/build-rspack-dev-tests-manifest.js
node test/build-rspack-build-tests-manifest.js
```
