pkg_run_script: use pkg->dest in half installed case

pkg_run_script: use pkg->dest in half installed case

Fixes running scripts for packages in half-installed state which would
otherwise fail with:

 * pkg_run_script: Internal error: ... has a NULL tmp_unpack_dir.
 * opkg_remove_pkg: not removing package "...", prerm script failed

Upstream bug: https://bugzilla.yoctoproject.org/show_bug.cgi?id=11245

Signed-off-by: Alejandro del Castillo <alejandro.delcastillo@ni.com>
[Jo-Philipp Wich: rebased onto opkg-lede, updated commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>