Use Ninja on Windows

MSBuild is slow, and its output is hard to parse visually
(it's all interwoven, like Make without `--output-sync`).
Since we use CMake anyway, we don't care about the build system.

.github/scripts/msvc-env.bat

@@ -0,0 +1,4 @@
+@rem From https://github.com/microsoft/vswhere/wiki/Start-Developer-Command-Prompt#using-batch
+for /f "delims=" %%p in ('vswhere.exe -prerelease -latest -property installationPath') do set vspath=%%p
+@rem From https://learn.microsoft.com/en-us/cpp/build/building-on-the-command-line?view=msvc-170#developer_command_file_locations
+call "%vspath%\VC\Auxiliary\Build\vcvars%1.bat"