Don't bother pinning OS images for Github Actions

[joerick]

Fix #1612

The hassle of updating these pins (both for us and our users, most copy our documentation) seems to outweigh the benefit of the pin. The truth is that they're not really pins anyway, Github update them all the time, and they don't work for historical repeatability because old images are retired all the time.

[henryiii]

ubuntu-latest is a good idea (we don't use anything really here anyway), but we probably should keep macos-* specified so that the images don't turn in AS by surprise. Once that does happen, then it's fine to use macos-latest (for AS).

[joerick]

ah yeah, good point. I'll make that change.

[joerick]

Thanks for doing that merge.