Unit tests fail when running against net6.0

[jeffhandley]

Running unit tests against the main branch with the projects targeting net6.0 is leading to a failed test.

 Libraries.Tests.PortToTripleSlashTests.Port_Basic
   Source: PortToTripleSlashTests.cs line 15
   Duration: 1 sec

  Message: 
    System.Exception : Diagnostic errors found.
  Stack Trace: 
    ToTripleSlashPorter.CheckDiagnostics(MSBuildWorkspace workspace, String stepName) line 479
    ToTripleSlashPorter.GetOrAddProject(MSBuildWorkspace workspace, Dictionary`2 projects, String projectPath) line 420
    ToTripleSlashPorter.GetProjectInfo(String projectPath, Boolean isMono) line 369
    ToTripleSlashPorter.Port() line 142
    ToTripleSlashPorter.Start(Configuration config) line 127
    PortToTripleSlashTests.PortToTripleSlash(String testDataDir, Boolean save, Boolean skipInterfaceImplementations, String assemblyName, String namespaceName, String typeName) line 54
    PortToTripleSlashTests.Port_Basic() line 17

The diagnostic error is:

"Failure - Msbuild failed when processing the file 'C:\\Users\\jeffhand\\AppData\\Local\\Temp\\0shltoqo.fn2\\Project\\MyAssembly.csproj' with message:
The expression \"[MSBuild]::GetTargetFrameworkIdentifier(net5.0)\" cannot be evaluated. MSB0001: Internal MSBuild Error: A required NuGet assembly was not found. 
Expected Path: C:\\Program Files\\dotnet\\sdk\\6.0.100-preview.2.21158.2  C:\\Program Files\\dotnet\\sdk\\6.0.100-preview.2.21158.2\\Sdks\\Microsoft.NET.Sdk\\targets\\Microsoft.NET.TargetFrameworkInference.targets"