Cancel previous builds within the same PR on push

[npaun]

What this does:

• Makes the test workflow responsible for triggering fixup, labels and lint.
• Cancels the entire workflow if another commit is pushed to the same PR
• Allows all other builds to run through.

This should cut down on the number of runners in use, and lead to less wait time for builds.

[danlapid]

I like the idea but we need to make sure we aren't reducing our concurrency at all
I don't want a full CI run to be synchronous now.
I think we just want it to be auto cancelled
You don't need the workflow for that, simply set the group to include ${{ github.workflow }} and they'll be fine.
Other than that we still need to make sure matrix jobs have different groups from each other
That might need to happen at the .concurrency.group level though.

[npaun]

This won't reduce our concurrency; the jobs still run in parallel. The advantage of the reusable workflow, where I was able to do to that, is just that I don't need to copy paste the concurrency group name into each workflow.

[npaun]

Just to be sure I grabbed this screenshot of a bunch of jobs running at once.