Qemu test: use pmbootstrap --details-to-stdout

The qemu test executes a pmbootstrap instance with pmb.helpers.run.user(). Use the --details-to-stdout flag from now on for the started pmbootstrap process, so it is not silent when downloading packages with apk, and will therefore not run into the timeout of the parent process (which kills processes that are silent for more than five minutes by default).

This should fix #1780 (closed). I'll wait until it runs through in CI, then merge it.