mkDerivation: Fix cross compilation

Derivations where drawing their `system` attribute from `hostPlatform`
instead of `buildPlatform`. Fix that, and add an explanatory commment.

Fixes #45993

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;