command/0.12upgrade: honor user-supplied plugin-dir

Fixes #21594

command/012_config_upgrade.go

@@ -56,6 +56,12 @@ func (c *ZeroTwelveUpgradeCommand) Run(args []string) int {
 		return 1
 	}
 
+	// Check for user-supplied plugin path
+	if c.pluginPath, err = c.loadPluginPath(); err != nil {
+		c.Ui.Error(fmt.Sprintf("Error loading plugin path: %s", err))
+		return 1
+	}
+
 	dir = c.normalizePath(dir)
 
 	sources, err := configupgrade.LoadModule(dir)