Fixes#97. In essence, the problem is that some packages in Nixpkgs have
RPATHs pointing to both $NIX_BUILD_TOP and $out, e.g.:
/tmp/nix-build-openldap-2.4.44.drv-0/openldap-2.4.44/libraries/libldap_r/.libs
/tmp/nix-build-openldap-2.4.44.drv-0/openldap-2.4.44/libraries/liblber/.libs
/nix/store/bfkmdxmv3a3f0g3d2q8jkdz2wam93c5z-openldap-2.4.44/lib
/nix/store/bfkmdxmv3a3f0g3d2q8jkdz2wam93c5z-openldap-2.4.44/lib64
Currently, running `patchelf --shrink-rpath` does the wrong thing by
keeping the /tmp/ paths and deleting the /nix/store ones. Now we can fix
the problem by using
patchelf --shrink-rpath --allowed-rpath-prefixes $NIX_STORE_DIR
in the Nixpkgs fixupPhase instead.
