Add new job var CYLC_WORKFLOW_NAME_BASE.

[hjoliver]

These changes close #4977

As requested: did not add CYLC_WORKFLOW_NAME_DIR

Small compromise: I did not remove CYLC_WORKFLOW_NAME because taken together it's immediately clear what the new variable is the basename of.

So now we have:

• CYLC_WORKFLOW_ID - e.g. a/b/c/run1
• CYLC_WORKFLOW_NAME - e.g. a/b/c
• CYLC_WORKFLOW_NAME_BASE - e.g. c

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

[wxtim]

I've assigned myself to do a code review so that @dpmatthews doesn't have to, and so that @oliver-sanders only has to do a second review.

[wxtim]

• Read
• Checked out locally, and successfully run the changed tests.
• 1 broken test - suggested fix in comment.
• Thank you for getting rid of all those ${CYLC_WORKFLOW_NAMES}.

[wxtim]

See comments.

[oliver-sanders]

Opened docs PR - cylc/cylc-doc#506

[hjoliver]

Docs PR merged. One review OK for this.