makechroot: download_sources: Accept makepkg_owner as an argument

What this is really doing is fixing a conflict that I had incorrectly
resolved when rebasing what became 2fd5931a onto cda9cf43.  Of course,
because of dynamic scoping, everything worked out, and everything worked as
intended.

Before cda9cf43, it was appropriate for download_sources to take src_owner
as an argument, but after cda9cf43, it is now appropriate to take
makepkg_user as an argument.  However, it still takes src_owner as an
argument, but pays 0 attention to it; instead looking at makepkg_user which
it happily inherited because of dynamic scoping.

So change it to take makepkg_user as the argument.