Fix environment handling in async version

[MonoidMusician]

The sync version was using the parsed env correctly, but it didn't make it into the async version which meant that extendEnv did not work when env was set.

To reproduce:

execa "env" [] _ { env = Just mempty, extendEnv = Just true }

would return only

NODE_CHANNEL_FD=3
NODE_CHANNEL_SERIALIZATION_MODE=json

[JordanMartinez]

Thanks. Good catch!

[JordanMartinez]

There's a number of other differences with execaSync that should also be updated. I'll push a fix for those as well.