MDEV-8532 MTR cannot run with --embedded on Windows on a source build

Do not attempt to run unix-specific code on Windows

mysql-test/mysql-test-run.pl

@@ -2686,7 +2686,9 @@ sub setup_vardir() {
   {
     $plugindir="$opt_vardir/plugins";
     mkpath($plugindir);
-    if (IS_WINDOWS && !$opt_embedded_server)
+    if (IS_WINDOWS)
+    {
+      if (!$opt_embedded_server)
       {
         for (<$bindir/storage/*$opt_vs_config/*.dll>,
              <$bindir/plugin/*$opt_vs_config/*.dll>,
@@ -2697,6 +2699,7 @@ sub setup_vardir() {
           set_plugin_var($pname);
         }
       }
+    }
     else
     {
       my $opt_use_copy= 1;