Fix including local packages.

Previously, including packages in config/chroot_local-packages would break the Tails build. The reason is that the version of live-build used by Tails tried to create an OpenPGP key to sign the local packages, but that key creation was designed with GnuPG 1.4.x in mind. With 2.x, GnuPG tries to invoke a pinentry tool to ask for a passworod, which failed likely due to stdin not being connected to a terminal.

This patch drops the key generation, and instead marks the local apt repository as trusted. This is what contemporary versions of live-build do.

Fixes tails#15618 (closed)