package_deb, apt.conf: fix apt failure in do_rootfs()

package_deb, apt.conf: fix apt failure in do_rootfs()

The changes in this commit, dc5f6c3898555b59f16bf809ae4c5418656e6ac9,
moved apt config directory from native SYSROOT to WORKDIR.
Unfortunately, Dir::Etc in apt.conf was not changed accordingly and
sources.list file could not be found during do_rootfs().

This commit fixes this issue.

[YOCTO #5241]

(From OE-Core rev: 65a2a1e1d0b0e3dcc34d47f46185f24d753b53dd)

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>