The reason for this change is explained in the long comment I added.
Here's a simple example of the problem:
let
pkgs = import <nixpkgs> { crossSystem.system = "aarch64-linux"; };
in
pkgs.callPackage ({ stdenv, s6-rc }: stdenv.mkDerivation {
name = "s6-rc-compiled";
nativeBuildInputs = [ s6-rc ];
buildCommand = ''
mkdir in
s6-rc-compile $out in
'';
}) {}
We're cross compiling for aarch64 here, so we'd expect the scripts
generated by this derivation to be things we could run on aarch64.
But when I build this on my x86_64 machine, without this change
applied, $out/servicedirs/s6rc-oneshot-runner/run gets generated full
of references to x86_64 non-cross store paths for execline, s6, and
s6-rc.
With this change applied, the scripts generated by the above
expression now refer to the cross-compiled aarch64 store paths for
execline, s6, and s6-rc.
6e6c8b7338