compatible with go workspace mode

[dydysy]

go: -mod may only be set to readonly or vendor when in workspace mode, but it is set to ""

[mvdan]

Thanks - I was trying to reset the -mod flag to its "unset" value, but as you point out, technically the empty string is not a valid value. The default behavior is "readonly" so I think this change is right. Since the change follows the docs, I don't think we need a test covering workspaces.