Unfortunately, the Mendix “Open Link” action is not able to open a link in the same tab. To solve your problem, this is exactly what you want. First: it seems that you opened your application to anonymous users, do you really want that? You can just show the login link on the login.html, wouldn't that be an option?
In order to open a link in the same tab, you need a widget to do this.
Option 1: use the HTML Snippet widget and put something like this in:
<a href="/signin/google">click here</a>
Option 2: use the Clickable Container widget and point to /signin/google