Add CYLC_SHARE_CYCLE_DIR to task script.

[wxtim]

Closes #6098

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 if present).
• Tests are included (or explain why tests are not needed).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[ColemanTom]

Just as a note. If people don't use rose task-env, nor the cycle directory, and never have, then this will be creating a folder that they now have to cleanup else risk leaving lots of empty folders on disk, using up potential inode quotas or having too many files on lustre in a single folder, causing performance degredation.

[oliver-sanders]

Yes, this will create one folder per-cycle. This doesn't seem particularly concerning to me given that we are already creating one folder and four-six files per-job. As a rule of thumb count(jobs) >> count(cycles) so I had not considered that this would be flagged as a performance issue.

However, if you have an existing housekeeping arrangement for log/job/<cycle>, then this might force inclusion of share/cycle/<cycle> in that arrangement (note rose_prune can manage this, one day Cylc may be able to do this itself). WDYT?

[elliotfontaine]

If I may put my two cents in, this would be really great! I use Rose anyway for the rose-suite.conf file, but this should be Cylc functionality as @oliver-sanders said in May. Right now I have $(eval rose task-env) in my [[root]] task, which is kinda weird.