Change pr-builder runner from ubuntu-latest to ubuntu-slim

[bdice]

GitHub’s ubuntu-latest runners are heavily backlogged, and pr-builder cannot run, which blocks merges. Switching to use self-hosted CPU runners should help alleviate the problem.

[gforsyth]

Do we still want to do this?

@jakirkham mentioned we've had our limit bumped for those runners last night at ~9pm eastern: https://nvidia.slack.com/archives/C01PF84NXCL/p1787186487740469?thread_ts=1787181186.430319&cid=C01PF84NXCL

[jameslamb]

I just asked our GitHub contacts a clarifying question... pr-builder might be a good fit for ubuntu-slim runners: https://github.blog/changelog/2025-10-28-1-vcpu-linux-runner-now-available-in-github-actions-in-public-preview/

Those are physically smaller and we might face higher concurrency limits for them... if so I'd rather switch to that than put more burden on our self-hosted CPU runners.

[bdice]

The reason I chose self-hosted infrastructure is because it isolates us from the recurring issues in GitHub infrastructure. I'll try ubuntu-slim, please approve and merge if you are fine with that.

[jameslamb]

Now that that outage is resolved and our ubuntu-latest concurrency limit has been increased, I think this should just be closed.

Our GitHub contacts said that ubuntu-slim and ubuntu-latest compete for the same concurrency, so unfortunately switching to ubuntu-slim doesn't help us.

Our self-hosted runners are plentiful but they do cost money and aren't truly unlimited.