Use property function instead of $(OS) (#5916)
Use property function instead of $(OS) (#5916)

This is more resilient to having the environment variable OS set.
AndyGerlicher/msbuild,sean-gilliam/msbuild,rainersigwald/msbuild,mono/msbuild,rainersigwald/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,mono/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,mono/msbuild,mono/msbuild