test: Stop hardcoded Arch repository/mirror
test: Stop hardcoded Arch repository/mirror

The `america` mirror has been down for several days, and makes all our
tests red. We have a reasonably current image now, so current packages
work. This could always happen again, but the proper way to fix this is
to provide a pre-cached build environment on our image.

cockpit-project/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,martinpitt/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit