[infra] Convert Kokoro build from Google Test to GitHub Action

`kokoro-mac` is configured as a workflow for PRs and commits. However, it's only internally visible, it points to destinations like `fusion2/ci/kokoro/prod%3Aflutter-intellij-kokoro%2Fmacos_external%2Fpresubmit/activity/fdb14539-3cb4-4e14-9208-9a0e6c036025` which is inaccessible from outside (of Google).

## Proposal

Setup a GitHub workflow instead, since the only matter is `plugin make --channel=dev`, which can be done like this: https://github.com/flutter/flutter-intellij/actions/runs/5888113417/job/15968672906?pr=6871.