Enable signing validation on non-Windows shipping VMR outputs

[ellahathaway]

Enable signing validation in the official VMR pipeline for non-Windows shipping outputs.

We will have to include at least one mac validation leg. This is due to the pkg tooling only being available on mac. For linux artifacts, we may be able to verify those shipping outputs on mac as well.

[ellahathaway]

This is blocked until infra is in place to run SignCheck on core. This may also be blocked on work to verify non-windows shipping artifacts.

[ellahathaway]

Blocked until we rebootstrap to a newer SDK that consumes the SignCheck .NET Core changes.