1. Use flashes.

A decent flash setup will overpower almost any artificial light, unless they are pretty powerful, like a football stadium. But besides that, it will.

It will also overpower almost any indirect sunlight.

You can use monolights, but even portable speed lights will, depending on how close your lights are to the subject. But for a headshot, it's no problem even at half-power or less.

There are decent low-cost brands. Try to get at least 2-3 units depending on the style of the headshot.

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2. Use the lights in your favor

You could diffuse it with a translucent nylon cloth. There are collapsable diffusers, but you need an assistant, or you could get a frame with light stands.

https://www.google.com/search?client=firefox-b-d&q=diffuser+for+sunlight

You could even make a PVC pipe frame.

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3. Just block it

If you are not superstitious, you can even use an umbrella! But you could use a boom stand with a collapsable reflector.

In some worst-case scenarios, where you have a window with direct sunlight you can use two light stands, like the ones that hold a backdrop, and put a black cloth.

4. Cardboard and tape

Or if you are really on a budget, some newspapers or cardboard and tape will do... No one uses newspapers anymore... Probably carry your own black cloth.