As-is we force it to be something like "x86_64"
instead of "le64" generally used on this platform.

This causes our software to interact badly with software
using the other (default!) values.
Small bonus: all le64 platforms can use same cache, maybe.

Motivation for this change

I'm not sure this was ever intended to be set twice (!)
as we presently are doing, looks like unintended result
of merging in some simplifications re:crossAttrs usage?

Anyway, cross or not, lets set this to the same values
the rest of the Linux world uses so we interop better.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.