package_manager/deb: Fix image generation with package removal

package_manager/deb: Fix image generation with package removal

When building SDKs with the deb backend you could see errors like:

Setting up nativesdk-python3-ndg-httpsclient (0.5.1-r0) ...
mkdir: cannot create directory ‘/usr/lib/opkg’: Permission denied
dpkg: error processing package nativesdk-python3-ndg-httpsclient (--configure):

which is due to environment misconfiguration when removing packages.

Fix this by setting the same environment variables as used for installation.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>