Initialize plugins after handling initial command line flags

This is technically a breaking change, since attempting to set plugin
files after the first non-flag argument will now throw an error. This
is acceptable given the relative lack of stability in a plugin
interface and the need to tie the knot somewhere once plugins can
actually define new subcommands.

src/libstore/globals.cc

@@ -253,6 +253,7 @@ void PluginFilesSetting::set(const std::string & str, bool append)
 
 void initPlugins()
 {
+    assert(!settings.pluginFiles.pluginsLoaded);
     for (const auto & pluginFile : settings.pluginFiles.get()) {
         Paths pluginFiles;
         try {