CI: add win64 targets built with ucrt
These are Win10 builds with a maximum API support of Windows 1806 = Redstone 5.
We use the LLVM "UWP" image as it's the one with UCRT runtime by default. It's not tied to UWP API's only.
Only the C runtime is different compared to the win64 LLVM builds we already have.
Fix the few hardcoded WINVER
that bypass the one we pass in the command-line (with an outdated one).
Edited by Steve Lhomme