[ArPow] job and step templates should provide a default managed platform

[MichaelSimons]

The source-build ArPow jobs template provides a defaultManagedPlatform if no platforms are specified. This makes calling the template easy if you want the "default" behavior. If a repo isn't able to utilize the Arcade jobs template, they have to call into the source-build job or steps template. At these layers, there is no default platform. If a repos wants to use the "default" source-build platform (e.g. container), they must define this themselves. This becomes a maintenance issue if want to change the default platform used in the source-build legs across the various repos.

[omajid]

A hardcoded container image name also shows up here in mono/linker which invokes the source-build steps directly: https://github.com/mono/linker/blob/50901f9ba5c8672c2758577a61f45fea3cc6cd76/eng/azure-pipelines.yml#L90-L102