[submodule_update] Parse flags

This was omitted when migrating from google3 because flag parsing was
handled implicitly by the google3-specific `google.Init()` function.

This caused the tool to always think that no command was specified.

Bug: 320755417
Change-Id: Ic9ee8c1a48aeeaea7c27128bc38f85ddd43c40cf
Reviewed-on: https://fuchsia-review.googlesource.com/c/infra/infra/+/980372
Fuchsia-Auto-Submit: Oliver Newman <olivernewman@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>
Reviewed-by: Rob Mohr <mohrr@google.com>

cmd/submodule_update/main.go

diff --git a/cmd/submodule_update/main.go b/cmd/submodule_update/main.go
index 1d85826..c3f0ba3 100644
--- a/cmd/submodule_update/main.go
+++ b/cmd/submodule_update/main.go
@@ -30,5 +30,6 @@
 	subcommands.Register(&integrationUpdateCmd{}, "")
 	subcommands.Register(&submoduleUpdateCmd{}, "")
 
+	flag.Parse()
 	os.Exit(int(subcommands.Execute(context.Background())))
 }