Start-tasks: wait on xtriggers

[hjoliver]

Start-tasks (cylc play --start-task=ID) are important for workflow DR failover.

Currently they get force-triggered at start-up, to run immediately. They should be spawned via setting all prerequisites instead, to wait on clock- and xtriggers. (c.f. clock-triggered tasks at the beginning of the graph, for normal start up).

(This came up in NIWA ops Cylc 8 migration, and @dwsutherland has had to work around it).

Very small change (+3/-1 lines code change + tests) but fairly important so should go in 8.4 IMO.

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).
• Changelog 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.

[hjoliver]

Example:

[scheduler]
    cycle point format = %Y
[scheduling]
    initial cycle point = 2024  # current year
    [[graph]]
        P1Y = "@wall_clock => a"
[runtime]
    [[a]]

Normal start:

• cylc vip - 2024/a runs, 2025/a waits on the clock ✔️

Start-task, master:

• cylc vip --start=task=2025/a - 2025/a runs immediately despite the unsatisfied clock trigger ❌

Start-task, this branch

• cylc vip --start=task=2025/a - 2025/a spawns but waits on the clock ✔️

[dwsutherland]

LGTM.. Much needed thanks 👍