Re-enable build/tests on macOS/linux

[kzu]

When bumping to NET6, the new dotnet (SDK? CLI?) fails to run tests that target net472 due to MSBuild internal implementation details/changes:

Can't find custom attr constructor image: /home/runner/.dotnet/sdk/6.0.300/Microsoft.Build.dll mtoken: 0x0a000007 due to: Could not resolve type with token 01000009 from typeref (expected class 'System.Resources.NeutralResourcesLanguageAttribute' in assembly 'System.Runtime, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a') assembly:System.Runtime, Version=6.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a type:System.Resources.NeutralResourcesLanguageAttribute member:(null)

Bumping tests to net6.0 caused a never ending stream of issues, where MSBuild wouldn't even resolve properly anymore and ALL tests in ALL platforms were broken. Therefore, as a stop-gap measure, I disabled the builds on on non-win OS for now.