fuzz: try setting PYTHONNOUSERSITE=1 to avoid loading site-packages

Authored by durin42.

Description

fuzz: try setting PYTHONNOUSERSITE=1 to avoid loading site-packages

Right now the fuzzer is crashing trying to look up the home dir for
uid 0, which is breaking in the fuzz environment.

Differential Revision: https://phab.mercurial-scm.org/D4936

Details