Introduce concept of env to make gpg signing easier to test about ghstack

[chebbyChefNEQ]

A few implementations come mind

1. Inject shell with env override for testing only

class Shell:
    def __init__(..., testing=true, testing_env_defaults={}):
        self.env_defaults = testing_env_defaults if testing else {}

2. Use https://pypi.org/project/pytest-forked/
   Then,os.environ pollution isolated. Kinda just a hack...

3. Plumb env thru <command>.main(env={...})
   Is this necessary? I don't see a use case of injecting env var other than testing.

WDYT?

[ezyang]

Is this necessary? I don't see a use case of injecting env var other than testing.

Isn't one use case good enough? :)