file: fix cross build for windows platforms (#476283)

pkgs/tools/misc/file/default.nix

@@ -2,7 +2,7 @@
   lib,
   stdenv,
   fetchurl,
-  file,
+  buildPackages,
   zlib,
   libgnurx,
   updateAutotoolsGnuConfigScriptsHook,
@@ -51,14 +51,16 @@ stdenv.mkDerivation (finalAttrs: {
 
   nativeBuildInputs = [
     updateAutotoolsGnuConfigScriptsHook
-  ]
-  ++ lib.optional (stdenv.hostPlatform != stdenv.buildPlatform) file;
+  ];
   buildInputs = [ zlib ] ++ lib.optional stdenv.hostPlatform.isMinGW libgnurx;
 
   # https://bugs.astron.com/view.php?id=382
   doCheck = !stdenv.buildPlatform.isMusl;
 
-  makeFlags = lib.optional stdenv.hostPlatform.isWindows "FILE_COMPILE=file";
+  # In native builds, it will use the newly-compiled file instead.
+  makeFlags = lib.optional (
+    !lib.systems.equals stdenv.hostPlatform stdenv.buildPlatform
+  ) "FILE_COMPILE=${lib.getExe buildPackages.file}";
 
   passthru.tests.pkg-config = testers.testMetaPkgConfig finalAttrs.finalPackage;