task_job_mgr: move compute out of open call

[oliver-sanders]

Spotted whilst reviewing #5475, writing to the activity log takes ~0.035 seconds per task which is ~8% of the main_loop runtime.

This PR reduces that to ~0.025s (~28% saving), not entirely sure why this is faster, but, meh.

I profiled this with:

# b = 1..100
a => <b> => c

Although you don't need 100 tasks and you could just time the method call.

Before:

After:

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.

[hjoliver]

Somewhat surprising that this makes a difference, but it's nicer this way anyway.