[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 #208 (the same change into release/5.x), then again on this PR into main:

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.

3. This PR into main. Opened with labels, milestone and body, then three remove/add pairs of a label: ten runs, five cancelled while waiting, five check runs on the commit, none cancelled, the last a success finishing last.

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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