build: special case building abuild

Since abuild is an implicit dependency of everything, we need special case handling for it. Insert it as a dependency of the first package in the build queue so that it will be built if outdated, then after building re-install it in the chroot so all subsequent packages will use the freshly built abuild.

This is currently used for systemd where we use an abuild fork

Signed-off-by: Caleb Connolly caleb@postmarketos.org