libhwy: fix tests when cross-compiling
libhwy: fix tests when cross-compiling

It seems to need some help to find gtest when cross compiling.

(Only applies when cross-compiling to compatible architectures where
the tests are enabled, like pkgsStatic.)

Co-authored-by: Daniel Olsen <ce4e6c3316bccb3adb9477f389b73f1c1a278f3c@gmail.com>

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs