Fix cat-log test timing out

[MetRonnie]

Follow-up to #6871

When cat-log is run in tail mode, and the log file is deleted or the tail process is killed, cat-log exits after 60s since #6871.

However, the test tests/functional/cylc-cat-log/12-delete-kill.t is waiting 60s for the cat-log process to exit so the test has become slow + flaky.

This fix allows customising the interval for watching the tail process via an env var CYLC_PROC_POLL_INTERVAL so that the test can be made to run quickly.

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).
• No dependency changes
• Tests are updated
• Changelog entry not needed
• Docs PR not needed
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[oliver-sanders]

Can't think of a better way!