Merge pull request #46024 from Ericson2314/fix-45993

mkDerivation: Fix cross compilation for 18.09

pkgs/stdenv/generic/make-derivation.nix

@@ -187,7 +187,15 @@ rec {
  builder = attrs.realBuilder or stdenv.shell;
  args = attrs.args or ["-e" (attrs.builder or ./default-builder.sh)];
  inherit stdenv;
- inherit (stdenv.hostPlatform) system;
+
+ # The `system` attribute of a derivation has special meaning to Nix.
+ # Derivations set it to choose what sort of machine could be used to
+ # execute the build, The build platform entirely determines this,
+ # indeed more finely than Nix knows or cares about. The `system`
+ # attribute of `buildPlatfom` matches Nix's degree of specificity.
+ # exactly.
+ inherit (stdenv.buildPlatform) system;
+
  userHook = config.stdenv.userHook or null;
  __ignoreNulls = true;