The build context inherits pkh's environment, so the rules ran with
HOME set to the invoking user's host path - which does not exist
inside the chroot. Tools keying user config on HOME then fail
mid-build: gdb's doc build dies in texi2dvi because the TeX live
setup cannot use the nonexistent home.
Point HOME at /tmp/pkh-home, created through the build context (all
drivers: local, unshare chroot, ssh, schroot). sbuild uses a
non-writable path, Launchpad's buildd user a real one; a writable
scratch directory matches the latter.