.github/workflows: cancel previous CI runs on PR update

Currently scheduled runs are not canceled when we update PRs, those
results are unused and the resources are just wasted. Instead of doing
that, this PR makes it so that when a PR is updated (force-pushed or new
commit added) the previous runs are canceled.

https://docs.github.com/en/actions/using-workflows/workflow-syntax-for-github-actions#example-using-a-fallback-value

Signed-off-by: Maisem Ali <maisem@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f