broadcast: fix dictionary changed size during iteration error

[oliver-sanders]

• Closes Broadcast - GraphQLLocatedError: dictionary changed size during iteration #6222

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.

[oliver-sanders]

)Replicating the conditions that trigger this is rather problematic, hence the lack of a test)

[hjoliver]

Nice 👍

[hjoliver]

(I don't think this needs multiple reviewers, but since you've deliberately assigned several I'll leave it unmerged for now).