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Fix the few hardcoded WINVER that bypass the one we pass in the command-line (with an outdated one).