How do I add a "Favorites" tab in Linux?

No, it is not.

Only way to get it at the moment is to merge it into a GitHub clone and configure/build/install it yourself.

Keep looking at the link I posted above, though. If that says merged instead of open it will be part of the available development build shortly, as in +/- 24h max., after. Not sure if you will still be on Linux, just in case: