[llvm] workflows: Rewrite build-ci-container to work on larger runners (PR #117353)

[Tom Stellard via llvm-commits]

tstellar wrote:

> Not too familiar with the depot runners. It seems like they're paid self hosted runners? Is the foundation paying for them?

The Foundation is providing a small budget for testing them out.  We're using 'Depot Managed' which means the instances are deployed in the Foundation's AWS account rather than using the machines from Depot's pool. 

> `llvm-premerge-linux-runners` should also work and will point to self hosted runners on the GCP project handling premerge. We intend to use that runner set to run the transitioned premerge jobs once we migrate to GHA.

Are these ready to use now?  I don't have a preference for which runners to use. 

https://github.com/llvm/llvm-project/pull/117353