Before calling `cmd.Execute` we need to reset the arguments to be used to not
include the global arguments which we have already parsed. This is precisely
the `args` which we have in our hand at this point.
This fixes `TestGlobalArgsOnlyParsedOnce/builtin` in the cli-plugins e2e tests.
`TestGlobalArgsOnlyParsedOnce/plugin` is still broken will be fixed next.
Signed-off-by: Ian Campbell <ijc@docker.com>