Optionally infer external references from CI environment

[nscuro]

https://circleci.com/docs/2.0/env-vars/#built-in-environment-variables https://docs.drone.io/pipeline/environment/reference/ https://docs.github.com/en/actions/reference/environment-variables#default-environment-variables https://docs.gitlab.com/ee/ci/variables/predefined_variables.html https://wiki.jenkins.io/display/JENKINS/Building+a+software+project#Buildingasoftwareproject-belowJenkinsSetEnvironmentVariables https://docs.travis-ci.com/user/environment-variables/#default-environment-variables