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

Merged Kristian Klausen requested to merge klausenbusk/arch-boxes:fast-single-thread-test into master

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

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