chore: Allow to change cwd of transcoder

[elthariel]

While using two pass encoders, it can be helpful to change the current working directory of the ffmpeg binaries.

This PR does just that by allowing to provide extra args to Open3 module functions.

PS: 3 tests are failing on my box, both on my branch and the up-to-date main branch, so I didn't take the time to investigate yet

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[elthariel]

Ok, there's a CLA now. Not sure I can sign it, I need to ask my company.

edit: I'm not sure I get why the CLA is required since the code is MIT ?