Summary:
GitHub Actions CI `windows-latest` environment has only VS 2019
installed, so we need to expand our logic to be able to locate it.

Note that Boost 1.69 doesn't know how to locate VS 2019 so we are effectively
tied to VS 2017 at the moment; the search order in this diff reflects that.

(That means that we can't target `windows-latest` on GitHub Actions, but that
is really a concern for a later diff in this stack)

Reviewed By: simpkins

Differential Revision: D17385052

fbshipit-source-id: 9bb0612154f42d425a625406488f39bb4ec3d8ae