Fixes issue

Fixes #864.

Describe the change

This allows environment variables to be added or removed from images.

How to verify it

regctl image mod nginx --env "[linux/amd64]NGINX_VERSION=1.27.3-custom" --create localhost:5002/library/nginx:custom
regctl image inspect --platform linux/amd64 localhost:5002/library/nginx:custom
docker run -it --rm localhost:5002/library/nginx:custom env

Changelog text

• Feat: Image mod environment variables.

Please verify and check that the pull request fulfills the following requirements

• Tests have been added or not applicable
• Documentation has been added, updated, or not applicable
• Changes have been rebased to main
• Multiple commits to the same code have been squashed