Use property function instead of $(OS)

[rainersigwald]

This is more resilient to having the environment variable OS set.

[benvillalobos]

There's also a use of $(OS) in Directory.Build.props on line 29.
rg -Fi "$(OS)"

src\Tasks\Microsoft.Common.CurrentVersion.targets
92:    <TargetPlatformSdkPath Condition="'$(TargetPlatformSdkPath)' == '' and '$(TargetPlatformIdentifier)' == 'Windows' and '$(OS)' == 'Windows_NT' and '$(MSBuildRuntimeType)' != 'Core'">$([MSBuild]::GetRegistryValueFromView('HKEY_LOCAL_MACHINE\Software\Microsoft\Microsoft SDKs\Windows\v$(TargetPlatformVersion)', InstallationFolder, null, RegistryView.Registry32, RegistryView.Default))</TargetPlatformSdkPath>

Directory.Build.props
29:    <TargetPlatformIdentifier Condition="'$(TargetPlatformIdentifier)'==''">$(OS)</TargetPlatformIdentifier>

Not sure where it's being set, as rg -Fi "<OS" doesn't gibe any relevant results.

[rainersigwald]

Yeah, I saw that one and didn't fully understand it so didn't mess with it. I think it might just be useless?

[benvillalobos]

Ideally we remove the other instance of $(OS) or get more context on it, but for the purpose of this PR I'm fine with merging.