Suggestion: `citgm-smoker-lite` job for PRs

This was briefly discussed in a Release WG meeting. A similar concept to https://ci.nodejs.org/view/All/job/node-test-commit-lite/, we could add a CITGM 'lite' job that only covers one or two most critical platforms (probably would make sense to be rhel8-x64 and osx11).

Running across all platforms takes a lot of time to both run, and interpret triage the results. Having a lite job running means we could increase our usage of CITGM on PRs. The more PRs we run it on, the earlier and easier it is to catch regressions, and may be easier for the wider collaborator base to keep up with the state.

(Perhaps eventually even a citgm-smoker-lite-daily job could make sense to cover the state over time - if there are volunteers to look at it.)

Releasers would still run CITGM and triage on the full set of platforms as part of the release process, but in theory they'd have an easier job as a number of issues are likely to have been caught by the lite job.

Start by reviewing the existing citgm-smoker job and the linked node-test-commit-lite job, focusing on how their platform selection and PR triggers are configured. Done would mean a documented citgm-smoker-lite job runs on PRs for the proposed rhel8-x64 and osx11 platforms, with results suitable for earlier regression detection.