[ci] Run Java setup in the background during build job setup (#37351)

In `build_and_lint`, `actions/setup-java` ran sequentially between
setup-node and the node_modules cache restore, but Java is only needed
by `yarn build` for the Closure Compiler bundles. This change marks the
setup-java step as a background step.

Setting up Java is mostly network (download) and CPU (unpack). It
overlaps with installing/restoring node_modules which is network and FS
work. So we aren't competing for resources that would make concurrently
running steps moot.

Co-authored-by: Claude Code (kimi-k3[1m]) <noreply@anthropic.com>