Fix Directory.build.props included in autogenerated projects.

Author: timcassell

tests/Directory.build.props

@@ -1,6 +1,6 @@
 <Project>
   <!-- ProjectReferences are not transitive, so we need to directly include the analyzers for all test projects. -->
   <ItemGroup Condition="'$(AssemblyName)' != 'BenchmarkDotNet.Analyzers.Tests'">
-    <ProjectReference Include="..\..\src\BenchmarkDotNet.Analyzers\BenchmarkDotNet.Analyzers.csproj" ReferenceOutputAssembly="false" OutputItemType="Analyzer" />
+    <ProjectReference Include="$(MSBuildThisFileDirectory)..\src\BenchmarkDotNet.Analyzers\BenchmarkDotNet.Analyzers.csproj" ReferenceOutputAssembly="false" OutputItemType="Analyzer" />
   </ItemGroup>
 </Project>