Unbreak the testsuite by supporting pacman 5.1

Created by: eli-schwartz

@pierres Is there a particular reason this was initially done via tmpfs?

Note that the specific error condition there will no longer be triggered by pacman 5.1.1 whenever that lands, but I still consider it sort of bad practice (plus I want to more closely mimic makechrootpkg itself). AFAICT docker creates a new container each time regardless.

If it's especially useful to ensure the /build directory is not persisted between runs (efficiency? I don't know enough about how docker works) I think it makes more sense to use makepkg --clean in the testsuite itself.