SharedFx tests in Helix always run against windows bits

[wtgodbe]

Tests in https://github.com/dotnet/aspnetcore/tree/main/src/Framework/test run in Helix, but always against bits built on Windows. We package a slightly different set of contents in the SharedFx on Windows vs non-Windows, so we don't actually have any automated test coverage ensuring that the non-windows SharedFx/Targeting Pack contain the assets that they should. We should figure out a way to get that coverage.

[dougbu]

nit: we may build the targeting packs when creating installers for multiple platforms. However, we don't test installers at all and those platform-specific targeting packs are not otherwise visible anywhere. We should fix the shared Fx gap before worrying about targeting packs.