add auto-cancellation of prior CI jobs when a PR is updated

In #4549 (closed) we moved from Travis CI to Github Actions. Travis would auto-cancel if a new commit was within a few seconds of a prior one. Appveyor does not do this, and it is a huge pain: see https://github.com/DynamoRIO/dynamorio/wiki/Test-Suite#ci-resources-and-appveyor-global-serialization

Looks like it might be possible to use someone else's support for auto-cancel out of the box: https://github.community/t/is-it-possible-to-cancel-previous-jobs-automatically-when-the-pull-request-is-updated/16779

If we also implement #4131 (closed) and move from Appveyor to Github Actions, maybe we could finally be free of queued up jobs and wasted CI resources.