[build-in-chroot.sh] Changelog:

* Show progress bar depending on whether or not the shell is interactive
* Disable signature verification in the chroot's pacman.conf until the package
  signing work is completed
* Remove existing packages in /var/cache/pacman/pkg/ with the same name as a
  newly built package (ie. when rebuilding a package). This is required so that
  pacman doesn't complain about mismatching sha256sums between ${LOCALREPO} and
  /var/cache/pacman/pkg/.