diffoscope: add native libraries to LD_LIBRARY_PATH

Reversal of global setting in previous commit necessitates
a local fix, otherwise, this happens:

  File "/home/pokybuild/yocto-worker/reproducible-debian/build/build-st-52142/tmp/work/x86_64-linux/diffoscope-native/172-r0/recipe-sysroot-native/usr/lib/python3.9/ctypes/__init__.py", line 392, in __getitem__
    func = self._FuncPtr((name_or_ordinal, self))
AttributeError: nativepython3: undefined symbol: archive_errno

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>