Kill tasks during job prep.

[hjoliver]

Close #5746

This PR allows cylc kill to abort jobs that are in the job preparation pipeline, resulting in the submit-failed state.

To avoid dealing with the complexities of partial job preparation, I've take the following approach:

• the kill command sets a flag in preparing task proxies
• the job submission process aborts at the last minute, after preparation has completed
• (so retriggering the task will result in a new submit number).

Tested by deliberately extending job preparation like this (and kill tasks whilst still in the preparing state)

platform = $(sleep 10; echo localhost)

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.

[MetRonnie]

After discussing removing a preparing task with Oliver (for #6472), this will probably be even more important as the task might end up submitting and running outside of the task pool if not killed? Not sure, haven't tested yet