Add trigger env

[SharpEdgeMarshall]

This PR adds the trigger author to the env vars available during the build

[CLAassistant]

All committers have signed the CLA.

[bradrydzewski]

Note that this code is deprecated and no longer being used by Drone. The runners are using the runner-go library to generate environment variables, which is creating the DRONE_BUILD_TRIGGER environment variable here https://github.com/drone/runner-go/blob/master/environ/environ.go#L153

[SharpEdgeMarshall]

@bradrydzewski then why is not indicated in the doc? https://docs.drone.io/pipeline/environment/reference/

[tphoney]

Hey @SharpEdgeMarshall i added the necessary documentation here https://docs.drone.io/pipeline/environment/reference/drone-build-trigger/
thanks for the effort on the PR, sorry for the wasted effort.