[CI] cancel in progress builds when a new push occurs

[cz4rs]

Make sure that older builds get canceled when new changes are pushed to a PR:

concurrency:
  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
  cancel-in-progress: true

DARMA-tasking/workflows#17 (comment)

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Also: use that issue to get rid of the dead links on the front page.