arpack: propagate isILP64 flag from BLAS

pkgs/development/libraries/science/math/arpack/default.nix

···
     install_name_tool -change libblas.dylib ${blas}/lib/libblas.dylib $out/lib/libarpack.dylib
   '';
 
+  passthru = { inherit (blas) isILP64; };
+
   meta = {
     homepage = "https://github.com/opencollab/arpack-ng";
     description = ''