vulkan-loader: fix cross-compilation

[Mindavi]

Description of changes

See issue KhronosGroup/Vulkan-Loader#881.

FIxup for update of vulkan(-loader): #170366.

Not 100% sure if this won't cause issues, but at least it fixes compilation for the package. It shouldn't change anything for the native build.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux (cross-compiled)
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

cc @sgillespie

[sgillespie]

This seems to fix vulkan-loader via:

 nix-build . --arg crossSystem '{ config = "aarch64-unknown-linux-gnu"; }' -A vulkan-loader

[Mindavi]

Hopefully this doesn't cause strange regresssions for cross, but this is the easiest solution for now, preventing having to build the native tools first and hacking up the build scripts.