Address more PR feedback

- Add libssl-dev dependency to DockerFile, because the package upgrade test still depend on this. It can be removed once a newer package without libssl-dev dependency is uploaded.
- Check pre-reqs in the package script.

scripts/dotnet-cli-build/Utils/Monikers.cs

@@ -33,7 +33,6 @@ namespace Microsoft.DotNet.Cli.Build
                     break;
                default:
                     throw new Exception($"Unknown channel - {channel}");
-                    break;
             }
 
             return packageName;