Fix bug in use of job runner command template.

[hjoliver]

These changes close #4987

Requirements check-list

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Applied any dependency changes to both setup.cfg and conda-environment.yml.

• [ ] Appropriate tests are included (unit and/or functional).
• [ ] Already covered by existing tests.
• [ ] Does not need tests (why?).

• [ ] Appropriate change log entry included.
• [ ] No change log entry required (why? e.g. invisible to users).

• [ ] (master branch) I have opened a documentation PR at cylc/cylc-doc/pull/XXXX.

• [ ] (7.8.x branch) I have updated the documentation in this PR branch.

• [ ] No documentation update required.

• [ ] [if bugfix] PRs raised to both master and the relevant bugfix branch.

[hjoliver]

The first commit fixes access to the global config item.

However, there's another problem, it's only used if the job runner has no built-in submit method.

DRAFT while I sort that out.

[hjoliver]

The dodgy logic: https://github.com/cylc/cylc-flow/blob/master/cylc/flow/job_runner_mgr.py#L689-L740

[MetRonnie]

Sorry, accidentally deleted 8.0.x!

[MetRonnie]

I'm closing this as it is not correct. job_runner_cmd_tmpl is consistently used as the key in the submit_opts dict

https://github.com/cylc/cylc-flow/blob/7e96003f6a0225cf135036bc6642dd4928499dbd/cylc/flow/job_runner_mgr.py#L688

https://github.com/cylc/cylc-flow/blob/c2bb31b9e396482d74a3f33b2c4fb80ef58577b5/cylc/flow/job_runner_handlers/documentation.py#L127-L135