Fix cross_compilation environment setting.

[mgautierfr]

We are setting LD_LIBRARY_PATH to point to the install dir to allow binaries (and test) to found our build library.

But when cross-compiling to native_dyn for musl (at least), python itself uses it and try to load libz (and such). But it cannot as libraries use different libc.
This is not a problem when we do a "full" cross-compiling as they are different arch. But here, we are same arch, different libc.