[libcxx] [llvm] [CI][Github] Add linux premerge workflow (PR #119635)

[Nathan Gauër via llvm-commits]

Keenuts wrote:

> > In our case we use GCP autoscale for scale up/down.
> 
> Yeah that's what I was using as well. I don't remember the customization needed, but that's all just Kubernetes under the hood right? And Kubernetes supports graceful scale down and draining down a service before decommissioning a pod I believe.

Yes, that was my understanding. I remember we had issues with pod getting killed, but it was because of spot instances or quota limits being reached, but not autoscale issues.
I'll follow up with Aiden to debug that

https://github.com/llvm/llvm-project/pull/119635