Use the fast fast-single-thread runner when using qemu for testing

The fast-single-thread runner is much faster (test-cloudimg-qemu):
3min 39sec[1] vs 8min 37sec[2]

[1] https://gitlab.archlinux.org/klausenbusk/arch-boxes/-/jobs/14739
[2] https://gitlab.archlinux.org/klausenbusk/arch-boxes/-/jobs/14746
6 jobs for fast-single-thread-test in 26 minutes and 56 seconds (queued for 1 second)
Status Name Job ID Coverage
  Lint
passed shellcheck #14776

00:00:10

passed shfmt #14777

00:00:07

 
  Build
passed build #14778
fast-single-thread

00:23:13

 
  Test
passed test-basic-qemu #14780
fast-single-thread

00:02:45

passed test-cloudimg-qemu #14781
fast-single-thread

00:03:31

passed test-vagrant-boxes-format #14779

00:00:36