`BOXEN_S3_BUCKET` is not loaded into the env

The consequences are that it's fine to `sync` our S3 with pre-built binaries because the script load the `config/boxen.rb` file. 

But when running `$ brew boxen-install package` is not okay anymore and it fallbacks to the boxen-downloads S3, and not our own S3...

Should we include the `BOXEN_S3_BUCKET` into the user env variables ?