Clean output directories for desktop frameworks when running on Linux
Clean output directories for desktop frameworks when running on Linux

Fixes #5497.

Note that this will still block us from using GitHub actions on
Windows, unless we can get more disk space or switch to "build+test"
one API at time, instead of building everything then testing
everything.

googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/gcloud-dotnet