Avoiding publish collisions

[therealryan]

#294 enabled the publishing of test reports from PR branches. This morning's bolus of dependabot PRs exposed a weakness - a bunch of the publish jobs failed as they were trying to update the pages branch at the same time, so there was a race condition where the losers failed on the git push - their commit history was suddenly out-of-date.

This change should mitigate that via the job-level concurrency control - now only one instance of the publish job should have access to the pages branch at a time,

[sonarqubecloud]

Kudos, SonarCloud Quality Gate passed!   

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