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