Check job not in prep already before triggering.

[hjoliver]

These changes close #4665

Don't retrigger a task that is already preparing for job submission.

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.
• Added test.
• Appropriate change log entry included.
• No documentation update required.

[hjoliver]

I'm not sure we can reliably/repeatedly test this. Tasks do not stay in the preparing state for very long. I've tentatively checked "covered by existing tests" because the code line changed is covered 🐑-ish.

[hjoliver]

(unrelated flaky test failure)

[oliver-sanders]

Looks good, makes sense.

This is the sort of thing we should be able to cover with an integration test, I'll take a look...

... done.

[MetRonnie]

Confirmed the new test fails on master, passes on this branch

[hjoliver]

Nice test, thanks @oliver-sanders