Migrate EARTHLY_* CLI variables to EARTHBUILD_

[JulesClaussen]

What existing functionality needs improvement?

Hello team,

Thank you for the huge amount of effort set in this! We're currently evaluating the migration, and noticed the CLI variables are still under EARTHLY_ (EARTHLY_ARG_FILE_PATH for example).
Would it be possible to migrate this towards EARTHBUILD_* (or EARTH_* ?)

Thank you!
Jules

Expected Behavior

Migrate EARTHLY_* env vars to EARTHBUILD_*

[janishorsts]

We need support EARTHLY_* for backward compatibility. However, we SHOULD print warning messages to users to update the environment variables to EARTH_* if any EARTHLY_* are found.

PS. migrate (autofix) command would be nice for a better experience. Not part of this ticket.

[kmannislands]

I believe this will be fixed by #384

@gilescope can you confirm? If so would be good to mark your PR as fixes #376