The following environment variables can be used to tweak CI behaviour and configuration. Some
of these may be ignored when config.environment.impure = false.
CI_CONFIG points to your CI configuration, if not provided via --arg config.
CI_CONFIG_ROOT points to your project root. This is used to resolve relative CI_CONFIG paths when generating cloud service configurations.
CI_PLATFORM indicates what environment the tests are currently running under. This should generally only be set when configuring a CI server, but the special impure value can be used to instruct the tests to use nixpkgs from your NIX_PATH environment. This may be useful when testing local changes or to avoid nix store bloat from using pinned channels.
TBD: CI_ALLOW_ROOT, CI_CLOSE_STDIN