Fix nss-localuser postinst scriptlet to run only on the target

Fix nss-localuser postinst scriptlet to run only on the target

Otherwise we fail on do_rootfs now.

Bug-AGL: SPEC-3385
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Change-Id: I80ff8c6eeec20d65de8bec770a1950bca3a43002
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/meta-agl/+/24529
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-boot-test: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Scott Murray <scott.murray@konsulko.com>