ci : add Arm Cobalt 100 runners

[ggerganov]

There are some new Github Actions runners "powered by the Cobalt 100-based processors":

https://github.blog/changelog/2025-01-16-linux-arm64-hosted-runners-now-available-for-free-in-public-repositories-public-preview/

Not sure what this processor is specifically, but it might have some Arm features that would be useful to exercise in the CI. We should look into more details and add workflows if it makes sense.