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

[Tom Stellard via llvm-commits]

tstellar wrote:

> > OK, do you think I should update this PR to use those runners instead or wait a few weeks?
> 
> I would say update it to use the new runners. This job doesn't run very often and probably shouldn't take that long to run on a decent sized machine.
> 
> > Depot runners are up-to 64 CPUs. The number of CPUs is part of the runs-on label so you can choose the right size machine that fits the job.
> 
> Ah, interesting. We don't have that much flexibility currently but could theoretically add the capability. Everything that I have seen/can imagine though either requires not much compute and should probably run on the free Github runners or requires a lot and would benefit from as many cores as possible.

Yeah, the nice thing about the builds is that they are highly parallel, so if you are paying for runners, you may as well use the one with the most CPUs, because the cost/job ends up being roughly the same no matter how many CPUs you have.

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