archbuild: local repositories not bound when chroot is updated

Created by: jrouleau

When pacman is run to update the chroot, local repositories are not bound causing pacman to fail

https://github.com/archlinux/devtools/blob/d8d2e23247a1aab5e269ff0e388e378ec9d462eb/archbuild.in#L79-L83

error: failed retrieving file 'custom.db' from disk : Couldn't open file [...]
error: failed to update custom (download library error)
 core is up to date
 extra is up to date
 community is up to date
error: failed to synchronize all databases
==> ERROR: Aborting...
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