Update build description on build cancellation

[kimonus]

Ghprb plugin has a feature of "Cancel build on update" which cancels current build in case if Pull Request being updated on GitHub. Unfortunately, I was not able to find a way from within current build to determine whether build was cancelled by ghprb or something else. So I suggest to add special string to build description -- to inform user/administrator that the build was canceled by this particular plugin.

Testing done

Plugin has been compiled and installed to Jenkins version 2.401.3.
And here is how it looks like:

1. In side panel:
   

2. In build details:
   

### Submitter checklist
- [X] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [X] Ensure that the pull request title represents the desired changelog entry
- [X] Please describe what you did
- [ ] Link to relevant issues in GitHub or Jira
- [ ] Link to relevant pull requests, esp. upstream and downstream changes
- [X] Ensure you have provided tests - that demonstrates feature works or fixes the issue