fix failed to build on windows

Author: ChanTsune

Diff for: src/Directory.Build.props

@@ -4,6 +4,9 @@
     <BUILD_OS Condition="'$([System.Runtime.InteropServices.RuntimeInformation]::IsOSPlatform($([System.Runtime.InteropServices.OSPlatform]::Windows)))' == 'true'">Windows</BUILD_OS>
     <BUILD_OS Condition="'$([System.Runtime.InteropServices.RuntimeInformation]::IsOSPlatform($([System.Runtime.InteropServices.OSPlatform]::OSX)))' == 'true'">OSX</BUILD_OS>
     <BUILD_OS Condition="'$([System.Runtime.InteropServices.RuntimeInformation]::IsOSPlatform($([System.Runtime.InteropServices.OSPlatform]::Linux)))' == 'true'">Linux</BUILD_OS>
+  </PropertyGroup>
+
+  <PropertyGroup Condition="'$([System.Runtime.InteropServices.RuntimeInformation]::IsOSPlatform($([System.Runtime.InteropServices.OSPlatform]::Windows)))' == 'false'">
     <BUILD_OS Condition="'$([System.Runtime.InteropServices.RuntimeInformation]::IsOSPlatform($([System.Runtime.InteropServices.OSPlatform]::FreeBSD)))' == 'true'">FreeBSD</BUILD_OS>
   </PropertyGroup>