Fix: Use env_cfg.to_dict() to avoid double JSON serialization (#43)

[vincentzhang]

This PR fixes issue #43 by replacing
json.dump(env_cfg.to_json(), fp, indent=4)
with
json.dump(env_cfg.to_dict(), fp, indent=4)
following the original suggestion.

[kevinzakka]

Thanks @vincentzhang !

[vincentzhang]

The failed tests seem to be related to the previous commit at 3c2583b
It seems to be a new MuJoCo feature introduced in google-deepmind/mujoco@cef5b31
which is not in the current MuJoCo release (3.2.7).

Scope of impact of this API change:
mujoco_playground/_src/dm_control_suite/reacher.py
and the unit test in
mujoco_playground/_src/dm_control_suite/dm_control_suite_test.py

[kevinzakka]

I'm waiting for the new MuJoCo release which should be out next week. We can merge this PR then?

[vincentzhang]

Good to know. Yeah I think this PR is good to go