remove runN if symlink target is gone

[wxtim]

These changes close #4323

• 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).
• Appropriate tests are included (unit and/or functional).
• Appropriate change log entry included.
• No documentation update required.

[kinow]

One test still failing @wxtim

=========================== short test summary info ============================
FAILED tests/unit/test_workflow_files.py::test_delete_runN[expect1-dirs1] - A...
====== 1 failed, 963 passed, 1 skipped, 1 xfailed, 12 warnings in 36.87s =======

[datamel]

Fast tests failing GH Actions. Looks to me like the same ones failing on #4361 which was merged into master.

These all pass locally for me. The plus is that bash / bash-docker (5.0) now passing.

[MetRonnie]

I guess there should also be a test that runN is not removed if cleaning a non-latest run dir

[MetRonnie]

Tested manually, all good 👍