Allow cylc clean to use runN

[MetRonnie]

This is a small change with no associated Issue. This fixes the bug:

$ cylc clean my-workflow/runN
WorkflowFilesError: Invalid symlink at ~/cylc-run/my-workflow/runN
The target should end with "cylc-run/my-workflow/runN"

Requirements check-list

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg and conda-environment.yml.
• Appropriate tests are included ( functional).
• Appropriate change log entry included.
• No documentation update required.

[datamel]

Checked out and tested. No problems found.