[Flax] Add jax flax to env command
This PR adds jax/flax libs to the env command.

@lewtun - can you maybe try out running `transformers-cli env` after this command to see why you cannot import `FlaxBigBirdForMaskedLM`
If most of the tests pass, then can I use the changes made by the PR in the time being to check whether I can atleast start my training?