Jenkinsfile: alternative to using git commit author for building workflow-cli image

[vdice]

As seen in #115 (comment), the assumption that the git commit author is the same as the user who owns the forked repo can be proven faulty. (Logic where this is used in CI to build a workflow-cli image with changes in a controller-sdk-go PR can be seen here)

We therefore seek an alternative way to fetch and inject a PR in this repo into a workflow-cli build artifact (via glide)