Set _IsPublishing=true in dotnet-aot.csproj evaluation

dotnet-aot.csproj should probably always be built for publishing. Doing so will avoid MSBuild task calls with global properties, as well as overwriting build artifacts with different versions depending on whether publish is being run or not.

This is the pattern used by finalizer.csproj.

See discussion on https://github.com/dotnet/dotnet/pull/6590 for more context. We can wait until the changes from that PR flow back into dotnet/sdk.