Optionally infer external references from CI environment

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

The issue names no files, tests, or entry points. Start by comparing the linked CircleCI, Drone, GitHub Actions, GitLab CI, Jenkins, and Travis CI environment-variable references, then inspect the repository to locate external-reference generation. Done would require a defined set of CI providers, variables, and expected inferred references, none of which the issue specifies.

Written by the indexing model from the issue text.