Investigate why CI fails often with TestCaptureOutput on

[nguerrera]

We started getting failures like this all of the sudden very often in CI (no repro locally):

F:\vsagent\10\s\.nuget\packages\microsoft.dotnet.arcade.sdk\1.0.0-beta.19065.6\tools\XUnit\XUnit.targets(93,5): error MSB3491: Could not write lines to file "F:\vsagent\10\s\artifacts\log\Release\Microsoft.DotNet.Configurer.UnitTests_netcoreapp3.0_x64.log". The process cannot access the file 'F:\vsagent\10\s\artifacts\log\Release\Microsoft.DotNet.Configurer.UnitTests_netcoreapp3.0_x64.log' because it is being used by another process. [F:\vsagent\10\s\test\Microsoft.DotNet.Configurer.UnitTests\Microsoft.DotNet.Configurer.UnitTests.csproj]

This is in a step in arcade where it appends to the log file that would have been written by a prior Exec with > redirected output to it. Somehow the file is still in use. Maybe exec returned before the handle was released? But how?