Constants for default OSes with pinned versions

[armanbilge]

In #762 we changed the default OS to a pinned Ubuntu version. Downstream builds which are using macOS and Windows may want to pin versions for those too for similar reasons. However, it's annoying if every downstream has to remember to bump these OS versions periodically. So maybe we should expose our recommended (and tested) pins versions as constants, than can be used in downstream builds.