Stop running tests on the mono runtime

Fixes #927

Author: AArnott

test/Nerdbank.GitVersioning.Tests/Nerdbank.GitVersioning.Tests.csproj

@@ -1,6 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net7.0;net472</TargetFrameworks>
+    <TargetFrameworks>net7.0</TargetFrameworks>
+    <TargetFrameworks Condition="$([MSBuild]::IsOSPlatform('Windows'))">$(TargetFrameworks);net472</TargetFrameworks>
     <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
     <GenerateBindingRedirectsOutputType>true</GenerateBindingRedirectsOutputType>
     <DebugType>full</DebugType>