ci: use github action concurrency to cancel jobs

[alexfikl]

Mostly inspired from
https://docs.github.com/en/actions/examples/using-concurrency-expressions-and-a-test-matrix
Basically should help with me pushing and rebasing in a quick succession that just generates a lot of jobs on the CI :(

[alexfikl]

@inducer Ok, something is wrong with that muller test :(
In #230, the only thing that changed there was the random number generator, so not sure what would annoy it :\ I'll try to run it locally to see if it ever triggers a failure.

[alexfikl]

@inducer Ran that test a few times and found a value that fails consistently in e13bc79. I haven't looked into what's wrong there though..