selftest/reproducible: Update config to match ongoing changes

We can't have systemd here any longer without usrmerge. We don't really
want to enable the latter since having separate usr will likely result in
a class of reproducibility and host contamination issues that enabling it
might hide.

Also drop INHIBIT_PACKAGE_STRIP since we generally don't build with that
and the debug binaries should be generated regardless. I suspect this
is legacy from older issues.

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