Don't use the secure runner on forks

The jobs will get stuck as the secure runner is only enabled for the
archlinux group.
7 jobs for runner-and-forks in 27 minutes and 36 seconds (queued for 1 second)
Status Name Job ID Coverage
  Lint
passed shellcheck #17324

00:00:18

passed shfmt #17325

00:00:11

 
  Build
passed build #17330
fast-single-thread

00:23:40

failed build #17326
fast-single-thread

00:00:45

 
  Test
passed test-basic-qemu #17328
fast-single-thread

00:02:50

passed test-cloudimg-qemu #17329
fast-single-thread

00:03:36

passed test-vagrant-boxes-format #17327

00:00:40