[MER-2347] No positional args. (#153)

Has no test code.

Arguably this warrants adding a test case that verifies we error on positional args to stack sync. 

I looked at our e2e_tests for a bit and didn't have an immediate idea of where/ how to add this and I'm not yet familiar with gittest. Feel free to bounce back to me that this needs a testcase.

cmd/av/stack_sync.go

@@ -48,6 +48,7 @@ latest commit to the repository base branch (e.g., main or master) into the
 stack. This is useful for rebasing a whole stack on the latest changes from the
 base branch.
 `),
+	Args: cobra.NoArgs,
 	RunE: func(cmd *cobra.Command, args []string) error {
 		ctx := context.Background()