feat: `clearEnv` on `CommandBuilder`

[Yohe-Am]

Solves #63.

Interplay between clearEnv and exportEnv proves tricky. I've went with a solution that, when both flags are set:

• Writes to the env will go through and affect the real env.
• Reads will not access any values acquired from the ambient environment.

[zifeo]

@dsherret Could this be reviewed? This support would help us to move forward with dax.