Remove redundant build actions from source-build.yml

In the arcade's build.sh, the `--source-build` switch already implies the restore, build, and pack build actions: https://github.com/dotnet/arcade/blob/6b63184d8ff276115846b232816ece935a26a22c/eng/common/build.sh#L133-L137

Arcade's `eng/common/core-templates/steps/source-build.yml` provides these actions _and_ the source-build switch. We should remove these redundant actions from the ymal.

We should also double check whether that's also true for fsharp, aspnetcore, roslyn and nuget-client's custom scripts as part of this work.