I am non-tenured and have been offered a tenure-track position at my current university, but I don't want to stay here long term. What should I do?

There's nothing to prevent you from accepting the TT position (which presumably carries better pay and more prestige) while still pursuing positions elsewhere. Indeed, having a tenure-track position may improve your chances of getting an offer outside, or may increase the leverage you have to bargain with if offered something elsewhere. And there is absolutely nothing wrong with accepting the TT job and then leaving (after a year) for another position. Your department head might not be happy, but this kind of thing is not viewed negatively by the academic culture at large.

In short, I see no downside for you to accept the tenure-track offer.

An answer to the general question (the one in your title):

Tenure track positions should be the default - they essentially mean you don't get arbitrarily fired. So, you've just been offered a regular position. Take it. If you later decide you want to leave, then you'll leave.

Adding the specific information about racism and harassment:

Well, one option you could consider is actively fighting against racism - in the university and in the community. As a TT you'll be in a better position to do this, as well as to promote and support others - students, graduate researchers, teachers (in the university) and public figures / community organizers (outside the university) when they do the same or when they're targeted. Now, I realize not everyone can/wants to shoulder this burden, so it's perfectly^H^H^H^H^H^H somewhat understandable if you decide not to. In that case - still, take the job. Look for another position on the side, and if one comes up - then you'll have a proper decision to make among two concrete alternatives.