makechrootpkg: fix potential non-writable directories of builduser in /build

This removes the preservation of HOME being /build just for the pacman
sudo call. Former leads to unbuildable packages when an to be installed
dependency writes something into the HOME dir (f.e. .config). The
resulting directories won't be writable by the builduser as they are
owned by root:root and ultimately will fail to build anything that
requires so.