pkgctl build: repo detection fails when there are dependency problems

Context/Motivation: https://bugs.archlinux.org/task/79544

$ pkgctl build --rebuild
==> Updating pacman database cache
:: Synchronizing package databases...
 core downloading...
 extra downloading...
 multilib downloading...
==> Building libredefender
error: failed to prepare transaction (could not satisfy dependencies)
==> ERROR: unknown repo, specify --repo for packages not currently in any official repo

This is because our repo detection fails due to the package having soname problems:

$ pacman --config /usr/share/devtools/pacman.conf.d/multilib.conf \
    --dbpath ~/.cache/devtools/pacman/db \
    -S --print --print-format "%n %r" libredefender

error: failed to prepare transaction (could not satisfy dependencies)
:: unable to satisfy dependency 'libclamav.so=11-64' required by libredefender