feat: add env-compact format.

[ya7010]

Currently, the env format of the load command passes an empty string for undefined environment variables.
This can cause applications to recognize the environment variable as existing.

As a countermeasure, I have added env-compact format that does not output undefined environment variables.

I considered adding --skip-undefined option, but since it would only affect env, so I have added a new format instead.

[changeset-bot]

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