[test] Method not found Xunit.Sdk.XunitException..ctor

Test: `AssemblyLoad_ProducesEvents`
Error Message: `System.MissingMethodException : Method not found: 'Void Xunit.Sdk.XunitException..ctor(System.String, System.String)'.`
Configuration: `Ubuntu_20_04_x64_Debug`

Stack Trace:
```
at Microsoft.DotNet.RemoteExecutor.RemoteExecutionException..ctor(String stackTrace)
at Microsoft.DotNet.RemoteExecutor.RemoteInvokeHandle.Dispose(Boolean disposing) in /_/src/Microsoft.DotNet.RemoteExecutor/src/RemoteInvokeHandle.cs:line 231
at Microsoft.DotNet.RemoteExecutor.RemoteInvokeHandle.Dispose() in /_/src/Microsoft.DotNet.RemoteExecutor/src/RemoteInvokeHandle.cs:line 57
at EventPipe.UnitTests.Common.RemoteTestExecutorHelper.RunTestCaseAsync(Action testCase, ITestOutputHelper output) in /__w/1/s/src/tests/eventpipe/common/RemoteTestExecutorHelper.cs:line 34
at EventPipe.UnitTests.LoaderEventsValidation.LoaderEventsTests.AssemblyLoad_ProducesEvents() in /__w/1/s/src/tests/eventpipe/LoaderEvents.cs:line 37
at System.Threading.Tasks.Task.<>c.b__128_0(Object state)
```

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=668134&view=ms.vss-test-web.build-test-results-tab&runId=16576362&resultId=100111&paneView=debug

PullRequest where test is failing https://github.com/dotnet/diagnostics/pull/4616 seems unrelated, probably a flakey test that hadn't failed in neighboring builds