bypass hubploy image building

[scottyhq]

GitHub Actions failing b/c we're bypassing image building (https://github.com/escience-pangeo/dssg-jhub/runs/694409160?check_suite_focus=true). Should be fixed once berkeley-dsep-infra/hubploy#75 is merged

[scottyhq]

For now, must deploy changes with helm locally:

git checkout staging
git pull
unset AWS_DEFAULT_PROFILE 
hubploy deploy dssg2020 hub staging