Logged spec now includes overridden args

[ojaffe]

Using --extra_eval_params will override args of the same name specified in the eval .yaml, but the updated values will not be logged in the spec, the original values will be logged instead. This PR fixes this problem; we just update eval_spec.args with the new values.

e.g. running oaieval dummy make-me-pay --extra_eval_params turn_cap=1 previously lead to "turn_cap": 5 being logged in the spec, since this is the default value.
In this branch, running the same command leads to "turn_cap": 1 being logged in the spec.

[JunShern]

Thanks @ojaffe! Agree that this is more intuitive behavior for users; have checked that this works as advertised.