selftest/reproducible: Allow native/cross reuse in test

selftest/reproducible: Allow native/cross reuse in test

We don't compare reproducibility of the native/cross components, only the
target ones. With the long build times of rust-native, the test now takes
crazy lengths of time so this tweak should allow us to reuse native/cross
artefacts from sstate whilst still testing the target output is
reproducible.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit b494d83c639a877cefeb7cbab6d37195e492f059)
Signed-off-by: Steve Sakoman <steve@sakoman.com>