buildRustCrate: fix target config environment variables on 32-bit ARM

pkgs/build-support/rust/build-rust-crate/default.nix

@@ -8,12 +8,6 @@
 , cargo, jq }:
 
 let
-    # This doesn't appear to be officially documented anywhere yet.
-    # See https://github.com/rust-lang-nursery/rust-forge/issues/101.
-    target_os = if stdenv.hostPlatform.isDarwin
-      then "macos"
-      else stdenv.hostPlatform.parsed.kernel.name;
-
     # Create rustc arguments to link against the given list of dependencies
     # and renames.
     #
@@ -52,7 +46,7 @@ let
    inherit (import ./log.nix { inherit lib; }) noisily echo_colored;
 
    configureCrate = import ./configure-crate.nix {
-     inherit lib stdenv echo_colored noisily mkRustcDepArgs mkRustcFeatureArgs;
+     inherit lib stdenv rust echo_colored noisily mkRustcDepArgs mkRustcFeatureArgs;
    };
 
    buildCrate = import ./build-crate.nix {
@@ -284,7 +278,7 @@ stdenv.mkDerivation (rec {
       inherit crateName buildDependencies completeDeps completeBuildDeps crateDescription
               crateFeatures crateRenames libName build workspace_member release libPath crateVersion
               extraLinkFlags extraRustcOpts
-              crateAuthors crateHomepage verbose colors target_os;
+              crateAuthors crateHomepage verbose colors;
     };
     buildPhase = buildCrate {
       inherit crateName dependencies