Continua Ci - uses registry - needs fix

[dazinator]

Originated from #1269:

I had to remove ContinuaCI support from NETStandard build - as lack of registry API's.

Why does it need access to the registry? It should just check if environment variables are registered, same as on Cake:

https://github.com/cake-build/cake/blob/08907d1a5d97b66f58c01ae82506280882dcfacc/src/Cake.Common/Build/ContinuaCI/ContinuaCIProvider.cs#L62

[dazinator]

PR submitted.