change config for sanity.cli.ts command line

[safejace]

with sanity v2 i was able to create .env.{something} and be able to run commands for specific environments

$ SANITY_ACTIVE_ENV=something sanity exec ...

with sanity v3 the project id and dataset are predefined and there are no docs about how to set the dataset in the command line

[englut]

If you use dotenv and process.env.SANITY_STUDIO_PROJECT_ID and process.env.SANITY_STUDIO_DATASET, you can re-use the same .env for both configs.