According to the docs we use java.io.tmpdir

https://docs.servicenow.com/bundle/london-servicenow-platform/page/product/mid-server/reference/mid-server-pre-upgrade-check.html

Seems like you can set this using either java opts or something in your bash_profile.  I don't know how you'd set this with the included java.  Perhaps putting this in one of the wrapper files.

-Dave