Motivation for this change

Side-stepping a problem with Nix-on-Droid.

Disables use of posix_spawn breaking on-device builds in Nix-on-Droid,
which currently piggy-backs on regular aarch64 nixpkgs builds.
https://discourse.nixos.org/t/9875

I understand this is a long shot, and generally not a very sane request,
but I'm desperate for options, and maybe it isn't that bad after all.

I'll totally understand if this won't be merged, but I thought it won't hurt to ask.

@vrthra

Things done

• [ x ] Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • [ x ] 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 nixpkgs-review --run "nixpkgs-review wip"
• [ x ] 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)
• Ensured that relevant documentation is up to date
• [ x ] Fits CONTRIBUTING.md.