Use environment variables in Jenkins UI pipeline

### What feature do you want to see added?

Is there a way to use environment variables in the Jenkins UI? When I try using `$BUILD_NUMBER` for example is doesnt do the string replacement. I also tried to join the Slack channel to ask this question but was unable.

### Upstream changes

_No response_

The issue names the Jenkins UI pipeline but no file, test, or entry point. Start by locating where this plugin defines its UI pipeline configuration and how `$BUILD_NUMBER` is processed; confirm the expected environment-variable behavior before changing it. Done means a pipeline value using `$BUILD_NUMBER` is substituted correctly and the behavior is covered by a test.

Written by the indexing model from the issue text.