Make `tox -e news` work on macOS

[sajith]

On macOS, tox -e news fails with this message:

WARNING: test command found but not installed in testenv
  cmd: /usr/local/bin/git
  env: /Users/sajith/tahoe-lafs/.tox/news
Maybe you forgot to specify a dependency? See also the allowlist_externals envconfig setting.

DEPRECATION WARNING: this will be an error in tox 4 and above!
Author identity unknown

*** Please tell me who you are.

Run

  git config --global user.email "you@example.com"
  git config --global user.name "Your Name"

to set your account's default identity.
Omit --global to set the identity only in this repository.

fatal: unable to auto-detect email address (got 'sajith@mac.(none)')
ERROR: InvocationError for command /usr/local/bin/git commit -m 'update NEWS.txt for release' (exited with code 128)

I do have user.name and user.email set in ~/.gitconfig. Looks like Towncrier needs $HOME in Tox news environment.