Only run the "build" job on runner with fast-single-thread

Some of our runners is vey slow and building on them takes +1 hour and
we need to adjust many of the timeouts, so let's just build on the fast
runners.
5 jobs for faster in 28 minutes and 23 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Lint
passed shellcheck #10945

00:00:13

passed shfmt #10946

00:00:13

 
  Build
passed build #10947
fast-single-thread

00:23:57

 
  Test
passed test-cloudimg-qemu #10949

00:04:12

passed test-vagrant-boxes-format #10948

00:00:44