nix: Patch config.nix.in to reference host platform binaries

[eamsden]

Motivation for this change

Presently the stable version of nix (2.3.3), when cross-compiled from nixpkgs, will pull into its closure and attempt to invoke the build platform's version of bash. This works around some of the build system for nix to correct that, so that a cross-compiled nix binary can be used on the system it is compiled for.

See #81153

Things done

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

[jonringer]

@GrahamcOfBorg eval

[flokli]

Can you also split the patching of config.nix.in and the cleanup into two separate commits, so we can only backport the fix into 20.03?

[eamsden]

@flokli I split into two commits and made sure no more tabs got in. Can you re-review?

[flokli]

I built this, and nix-store -qR $(nix-build -A pkgsCross.armv7l-hf-multiplatform.nix) seems to have references to native coreutils, glibc and whatnot, however, it seems to be an improvement.

Did you copy over the closure and see if nix behaves properly?

[eamsden]

I have not yet. Odd that the closure still pulls in native stuff. I'll try it tomorrow when I can get my pi back out