MAINT-8822: Revert viewer_manifest to using src= instead of build=.

Turns out that having multiple (source, build, artwork) prefix stacks isn't
such a good idea after all. Many of our LLManifest.path() directives use
wildcards -- and if _any files_ match the specified wildcard from a prefix
stack other than what you had in mind, viewer_manifest will silently,
cheerfully do the Wrong Thing.

There is a good reason why all existing LLManifest.prefix() calls exclusively
used src= and/or dst= instead of build=.