How to move session directory from /var/php_sessions/

I'm using a hosting provider that doesn't allow me to write into this directory, what do I need to modify to point this to a directory I can write to?

They don't let you use PHP sessions? That's all Grav is doing here, it's not asking for a specific directory, it's just asking PHP to create a session, and the PHP configuration dictates what folder that is going to use.

If that is causing an error, then your hosting provider needs to fix the PHP configuration and point it somewhere sessions can be created.