The NIX_LIB64|32_IN_SELF_RPATH environment variables control whether
to add lib64 and lib32 to rpaths. However, they're set depending
on the build paltform, not the target platform and thus their values
are incorrect for for cross-builds.
On the other hand, setting them according to the build platform introduce
pointless differences in build outputs; see #221350 for details.
This change fixes the issues by boldly removes the NIX_LIB*_IN_SELF_RPATH
facility altogether, in the hope that it is no longer necessary. They
were introduced in 2009, long before nixpkgs had good support for
cross-builds.
Fixes#221350
88fd6601d8