Combine SCL Build and Test Phase

[jacderida]

It seems to be either impossible or very difficult to stop SCL from being recompiled in separate Docker sessions, even if you are passing in the built artifacts. It won't recompile the dependencies, but it does attempt to recompile SCL itself. We can combine these 2 stages into a single build and test stage, and just upload the artifacts after that.

This should significantly reduce the build time.