[CaffeinatedCoder]

Closes #207

Problem

pr-hygiene.yml cancelled every run but the last within its concurrency group (cancel-in-progress: true): 152 of the 200 runs before this PR. "Labels and issue link" is a required check on release/5.x and main, and each cancelled run left a CANCELLED check run on the commit.

Change

Keeps the group and sets cancel-in-progress: false, so the runs of one PR go one at a time and none that has started is cancelled. The workflow's header comment records why, from the measurements below.

Tests

Measured on this PR, which runs its own copy of the workflow.

1. No group (first commit, d80aa07). Opened with labels, milestone and body in one gh pr create: four runs, all completed, none cancelled. Then the failure the issue warned of: a label removed and re-added gave a failing run and a passing one; re-running the failing run, so its old payload finished last, made FAILURE the check's latest result although the labels were in order. Without a group, runs finish in any order, and the last to finish decides.
2. Serialized (cb3145d). A push, then three remove/add pairs of a label within 4 seconds: seven runs. They ran in event order, the last event's run (passing) finished last at 14:58:48, and the three runs cancelled while waiting left no check run on the commit. The commit's "Labels and issue link" check runs: success, failure, failure, success, in that order, the last deciding.

So a cancelled run can still appear in the Actions list, but only one that never started, and never as a check run.

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