Pass matching arm64 arch to build-managed step

We need the arch to match on all build steps even if the
arch doesn't matter for the building of the code on that
step. It still needs to match so that the native dependencies
like the runtime are correctly being restored and used.

.NET Core 2.1 preview2

.Net Core 2.0.7

.Net Core 1.1.8

.Net Core 1.0.11

.NET Core 2.0.6

.NET Core 1.1.7

.NET Core 1.0.10

.Net Core 2.1 preview1

For .NET Core 2.0.5 release