Use bootstrap images when possible

After the problems we have experienced when trying to build AlmaLinux 9 kmods in AlmaLinux 10 with recent versions of mock, we have decided to start using bootstrapped chroots when building with mock.

Also, in order to speed up the build time a bit, we have decided to start using bootstrap images to the extent possible. The only exception is x86_64_v2, for which we can not provide an image for mock to use, we'll create the bootstrap from scratch.