ci: avoid unnecessary builds

Whenever a branch is pushed to a repository which has GitHub Actions
enabled, a bunch of new workflow runs are started.

We sometimes see contributors push multiple branch updates in rapid
succession, which in conjunction with the impressive time swallowed by
even just a single CI build frequently leads to many queued-up runs.

This is particularly problematic in the case of Pull Requests where a
single contributor can easily (inadvertently) prevent timely builds for
other contributors when using a shared repository.

To help with this situation, let's use the `concurrency` feature of
GitHub workflows, essentially canceling GitHub workflow runs that are
obsoleted by more recent runs:

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

For workflows that *do* want the behavior in the pre-image of this
patch, they can use the ci-config feature to disable the new behavior by
adding an executable script on the ci-config branch called
'skip-concurrent' which terminates with a non-zero exit code.

Original-patch-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

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