pause: workflow cannot be stalled if it is paused

[oliver-sanders]

closes #4243

Fixes bug where the stall detection could release runahead tasks.

Spotted by @MetRonnie on #4237 (comment)

Bug was intermittent, however, could be made dependable by jamming an await asyncio.sleep(1) into tests/i/test_scan::test_scan_sigstop.

The released can be seen an extra log message in the test output which was causing the failure:

E             Full diff:
E               [
E             +  (20,
E             +   '[one.1] -released from runahead'),
E                (30,...

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.py and
  conda-environment.yml.
• Appropriate tests are included (unit and/or functional).
• No change log entry required (why? bug never released)
• (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.

[oliver-sanders]

I don't fancy adding a dedicated integration test for this as that would involve putting a workflow into a stalled state then waiting around for an arbitrary period just to make sure it doesn't do anything.