How to set a Circle CI environment variable for a project?
https://circleci.com/docs/2.0/env-vars/#setting-an-environment-variable-in-a-project

Seems "contexts" can be used but must then also be set to "reference" the context in the build yaml config file.

Instead to set a environment variable for a project:
Jobs > Cog / gear icon of a project > Build settings > Environment variable