Simplify github actions CI yaml file to use stock github runners and latest versions of windows/mac/ubuntu (#48)

* simplified github actions to include just 8 tasks, 4 linux, 2 macos, 2 windows, always on the latest version of the os/runners that github gives us

* using specific sha when pointing to the actions/checkout repo instead of a tag, a tag can be moved around, but a sha can't so it's more stable

Author: grdanny