Introduce check to see if chroot version file exists

We currently unconditionally cat a file in archbuild:

CURRENT_CHROOT_VERSION=$(cat "${chroots}/${repo}-${arch}/root/.arch-chroot")

This results in the following error:

==> Updating pacman database cache
:: Synchronizing package databases...
 core downloading...
 extra downloading...
 multilib downloading...
==> Building protobuf
  ->   repo: extra-staging
  ->   arch: x86_64
  -> worker: chris-3
==> Building protobuf for [extra-staging] (x86_64)
cat: /var/lib/archbuild/extra-staging-x86_64/root/.arch-chroot: No such file or directory
==> Creating chroot for [extra-staging] (x86_64)...

This should either be properly guarded (if [[ -f ... ]]) or the error should be silenced 😊