D8021: chg: switch to using global `environ` instead of envp from main

[spectral (Kyle Lippincott)]

spectral added a comment.


  > I prefer passing in (const char**)environ to hgc_setenv().
  
  Done.

REPOSITORY
  rHG Mercurial

CHANGES SINCE LAST ACTION
  https://phab.mercurial-scm.org/D8021/new/

REVISION DETAIL
  https://phab.mercurial-scm.org/D8021

To: spectral, #hg-reviewers
Cc: yuja, pulkit, mercurial-devel