ci: .NET Framework test builds fail with .NET 8 SDK preview

[skwasjer]

Build exception:

Testhost process for source(s) '/home/runner/work/MockHttp/MockHttp/test/MockHttp.Json.Tests/bin/Release/net48/MockHttp.Json.Tests.dll' exited with error: Unhandled Exception:
System.TypeInitializationException: The type initializer for 'Microsoft.VisualStudio.TestPlatform.ObjectModel.EqtTrace' threw an exception. ---> System.TypeLoadException: Could not load type of field 'Microsoft.VisualStudio.TestPlatform.ObjectModel.PlatformEqtTrace:TraceSourceLevelsMap' (1) due to: Could not resolve type with token 01000019 from typeref (expected class 'System.Diagnostics.TraceLevel' in assembly 'System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089') assembly:System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 type:System.Diagnostics.TraceLevel member:(null)
   --- End of inner exception stack trace ---
  at Microsoft.VisualStudio.TestPlatform.TestHost.Program.Main (System.String[] args) [0x00033] in <985811b3ce274022a2072f51d4b88753>:0 
Error:  FATAL UNHANDLED EXCEPTION: System.TypeInitializationException: The type initializer for 'Microsoft.VisualStudio.TestPlatform.ObjectModel.EqtTrace' threw an exception. ---> System.TypeLoadException: Could not load type of field 'Microsoft.VisualStudio.TestPlatform.ObjectModel.PlatformEqtTrace:TraceSourceLevelsMap' (1) due to: Could not resolve type with token 01000019 from typeref (expected class 'System.Diagnostics.TraceLevel' in assembly 'System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089') assembly:System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 type:System.Diagnostics.TraceLevel member:(null)
   --- End of inner exception stack trace ---
  at Microsoft.VisualStudio.TestPlatform.TestHost.Program.Main (System.String[] args) [0x00033] in <985811b3ce274022a2072f51d4b88753>:0 
. Please check the diagnostic logs for more information.

Temp pinned to previous SDK version, until SDK installer is updated.

microsoft/vstest#4549

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