ci: switch to smaller ubuntu-slim runner image

[afuetterer]

Description

For these smaller ci jobs the newer ubuntu-slim runner should be sufficient, no need for a full fledged VM.

Refs:

• https://github.blog/changelog/2025-10-28-1-vcpu-linux-runner-now-available-in-github-actions-in-public-preview/
• https://docs.github.com/en/actions/reference/runners/github-hosted-runners

Motivation and Context

How has this been tested?

[MyPyDavid]

thanks @afuetterer , looks fine to with the -slim image.
Am not sure whether we can also take this already in the 2.4.5 patch release? @jochenklar

[jochenklar]

Yes, and I think we should even do this directly from the GitHub actions to PyPI. I think a less prominent release is better for this. I will prepare a PR and then you can check if I did it right.