fix codemodding on windows

[Josverl]

Summary

running codemodding fails due to passing an empty environment env={} rather than env=None according to the Python spec.
Note If specified, env must provide any variables required for the program to execute. On Windows, in order to run a side-by-side assembly the specified env must include a valid SystemRoot.
by specifying env=None things work smoothly on Windows 10

fixes: #494

Test Plan

I have only performed limited testing on Windows, but this should be validated across all supported platforms.
My assumption that this is already part of the CI/CD tests

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[zsol]

Thx for contributing! :)

[bgw]

After looking at the pre-OSS blame for this, it seems like the use of env={} in the first place was to work around an internal issue with an Instagram script runner. It's unclear if that issue has been resolved (though it probably has). Curious Facebook employees can look at D14541659.

[zsol]

Looks like that particular error has been resolved, but I see a different one today. In any case, we should fix this issue internally