Relative env-dir breaks tools

[henryiii]

Describe the bug

Nox currently sets TMPDIR to a relative path - this causes temporary directory creation to break for tools run after a session.cd. At least TMPDIR should be an absolute path - and maybe the return value from session.create_tmp should be absolute, too? (It really should be a pathlib.Path, but I'm referring to backwards compatible changes here ;) )

(see scientific-python/cookie#60 - Maturin expects TMPDIR to be an existent directory)

How to reproduce
Use chdir then try to run a tool that makes temporary directories.

Expected behavior
Temporary directory creation should not be broken after changing directories.