[MacPorts] #63974: GitHub CI: update PR workflow to cancel outdated jobs on push

[MacPorts]

#63974: GitHub CI: update PR workflow to cancel outdated jobs on push
----------------------------+---------------------
 Reporter:  mascguy         |      Owner:  admin@…
     Type:  enhancement     |     Status:  new
 Priority:  Normal          |  Milestone:
Component:  server/hosting  |    Version:  2.7.1
 Keywords:  github ci       |       Port:
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 Presently, whenever code is pushed for a PR, a new CI job is created.
 However, any previous job(s) continue to run (or remain queued), which
 wastes precious CI resources.

 As of this writing, GitHub Actions now appear to support the ability to
 automatically cancel previous jobs, when a subsequent push occurs.

 It's not 100% clear what's needed for our current workflow setup, but some
 Googling suggested the following:

 https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-
 for-github-actions#concurrency

 https://docs.github.com/en/actions/learn-github-actions/workflow-syntax-
 for-github-actions#jobsjob_idconcurrency

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Ticket URL: <https://trac.macports.org/ticket/63974>
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