Also include host rust

We need both rust versions to build anything useful.

rust-overlay.nix

@@ -119,7 +119,7 @@ let
       inherit (super.lib) flatten remove subtractLists unique;
       targetExtensionsToInstall = checkMissingExtensions pkgs pkgname stdenv targetExtensions;
       extensionsToInstall = checkMissingExtensions pkgs pkgname stdenv extensions;
-      hostTargets = [ "*" (self.rust.toRustTarget stdenv.targetPlatform) ];
+      hostTargets = [ "*" (self.rust.toRustTarget stdenv.hostPlatform) (self.rust.toRustTarget stdenv.targetPlatform) ];
       pkgTuples = flatten (getTargetPkgTuples pkgs pkgname hostTargets targets stdenv);
       extensionTuples = flatten (map (name: getTargetPkgTuples pkgs name hostTargets targets stdenv) extensionsToInstall);
       targetExtensionTuples = flatten (map (name: getTargetPkgTuples pkgs name targets targets stdenv) targetExtensionsToInstall);