feature_flags_SUITE: Use ";" as plugin path separator on Windows

(cherry picked from commit e60ac4f)

Author: dumbbell

test/feature_flags_SUITE.erl

@@ -879,7 +879,11 @@ build_my_plugin(Config) ->
     PluginSrcDir = filename:join(?config(data_dir, Config), "my_plugin"),
     PluginsDir1 = filename:join(?config(current_srcdir, Config), "plugins"),
     PluginsDir2 = filename:join(PluginSrcDir, "plugins"),
-    PluginsDir = PluginsDir1 ++ ":" ++ PluginsDir2,
+    PathSep = case os:type() of
+                  {win32, _} -> ";";
+                  _          -> ":"
+              end,
+    PluginsDir = PluginsDir1 ++ PathSep ++ PluginsDir2,
     Config1 = rabbit_ct_helpers:set_config(Config,
                                            [{rmq_plugins_dir, PluginsDir}]),
     case filelib:wildcard("plugins/my_plugin-*", PluginSrcDir) of