debootstrap: fix buildd variant not installing apt

apt in trisquel aramo is neither Build-Essential: yes nor Priority: required, causing it not being pulled when creating a buildd chroot. Since the build-essential package includes all packages that are Build-Essential: yes, use build-essential instead.

Also, the trisquel-common script is not referred to or run by any other programs, so it's useless. Remove it.

Fixes: 76a9f47e Fixes: a75b1ffc Signed-off-by: Jing Luo jing@jing.rocks (cherry picked from commit 98d79c33)