Added flag to skip 3rd party package_index in UpdateIndex gRPC API

Fix arduino#1788

Author: cmaglie

Diff for: cli/instance/instance.go

@@ -154,7 +154,8 @@ func FirstUpdate(instance *rpc.Instance) error {
 	if packageIndex.NotExist() {
 		_, err := commands.UpdateIndex(context.Background(),
 			&rpc.UpdateIndexRequest{
-				Instance: instance,
+				Instance:                   instance,
+				IgnoreCustomPackageIndexes: true,
 			},
 			output.ProgressBar(),
 		)

Diff for: commands/instances.go

@@ -498,7 +498,9 @@ func UpdateIndex(ctx context.Context, req *rpc.UpdateIndexRequest, downloadCB rp
 	indexpath := configuration.DataDir(configuration.Settings)
 
 	urls := []string{globals.DefaultIndexURL}
-	urls = append(urls, configuration.Settings.GetStringSlice("board_manager.additional_urls")...)
+	if !req.GetIgnoreCustomPackageIndexes() {
+		urls = append(urls, configuration.Settings.GetStringSlice("board_manager.additional_urls")...)
+	}
 	for _, u := range urls {
 		logrus.Info("URL: ", u)
 		URL, err := utils.URLParse(u)