Undocumented `$CYLC_WORKFLOW_LOG_DIR`

[MetRonnie]

CYLC_WORKFLOW_LOG_DIR needs adding to the job script environment variables reference: https://cylc.github.io/cylc-doc/latest/html/reference/job-script-vars/index.html

A quirk is that this points to ${CYLC_WORKFLOW_RUN_DIR}/log/scheduler rather than just ${$CYLC_WORKFLOW_RUN_DIR}/log - see cylc/cylc-flow#6494

Also, for its Jinja2 equivalent, if we don't change where it points to, we need to update the description here to reflect this quirk:

cylc-doc/src/user-guide/writing-workflows/jinja2.rst

Lines 156 to 162 in d76834f

	Additional Jinja2 CYLC variables available when the scheduler is parsing an
	installed workflow at run time:
	.. table::
	======================= ==============
	CYLC_WORKFLOW_LOG_DIR Workflow log sub-directory

[oliver-sanders]

(when addressing this issue, please go through all the env vars exported in the job.sh file to check there aren't other omissions)