getdeps: prefer MSVC over gcc when building on GitHub Actions CI
getdeps: prefer MSVC over gcc when building on GitHub Actions CI

Summary:
while testing https://github.com/facebook/watchman/pull/743 I
noticed that the cmake builds were picking up the installed mingw GCC
compiler rather than the MSVC compiler.  That would be fine except that
boost is built with MSVC and its generated libraries cannot be subsequently
found by a cmake gcc build that uses FindBoost.

This commit forces cmake to pick cl.exe rather than gcc.  This is probably
fine to do unconditionally on windows, but since I've only observed this
particular problem with GitHub Actions I'm keeping it constrained to that
environment for now.

Reviewed By: simpkins

Differential Revision: D17385050

fbshipit-source-id: 90bef898b138e5d4bbd28a155ed1bd468acee9de

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly