Use -smp 4 for qemu

Will more cores make the build faster? Let's find out.
3 jobs for try-smp in 17 minutes and 41 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Lint
passed shellcheck #5224

00:00:15

passed shfmt #5225

00:00:11

 
  Build
passed build #5226

00:17:25