update Positron API tests to use ubuntu runners

As follow up work to https://github.com/posit-dev/positron/issues/14531, once 2026.09 has been released, we should update the `mac-os` runners to `ubuntu-latest`. At the same point in time, let's make sure all the `setup-positron` github actions are on `v1`, not pinned to main, so if we ever want to make updates, we don't disturb folks' CI runs. Since these should all be 1-2 line changes, instead of splitting it out into 8 new issues again, we can just mark it off here. - [ ] SAS extension < link to PR once it exists > - [ ] SLC extension < link to PR once it exists > - [ ] Domino extension < link to PR once it exists > - [ ] `posit.publisher` extension < link to PR once it exists > - [ ] `posit.shiny` extension < link to PR once it exists > - [ ] Julia extension < link to PR once it exists > - [x] `posit.assistant` extension posit-dev/assistant#2080 - [ ] `quarto.quarto` extension https://github.com/quarto-dev/quarto/pull/1094