Cross-Compile rzc to target netcoreapp3.0

[pranavkm]

Roll-forward semantics work for minor versions but not across major versions. If a machine only has netcoreapp3.0 installed, running rzc would fail saying the runtime could not be found.

See discussion in: dotnet/aspnetcore#3698

[pranavkm]

Modified the compiler to target netcoreapp3.0. This aligns with the runtime carried by the 3.0 SDK