feat(tvix): Write even more build logs to client r/1767

Write *all* build logs (essentially everything that wasn't a DLOG), not
just the stdout of the builder, to the client-bound log_sink_ that's
threaded through build.cc. These logs are significantly more useful on
the client, both when debugging tvix itself and (more importantly) when
building things with it.

To make this all work, this commit also refactors the passing-around of
the log sink in build.cc slightly - the main reference is now owned by
the Worker class, which already stores pointers to itself on the Goals
it creates, so those Goals can then load the log sink from the Worker.
This change was originally made to allow for logging to the log_sink
from instance methods of the worker, but actually feels significantly
more natural than storing the sink on the goal anyway.

Change-Id: I461685bc7f5c0a326054940ac5971b568dd43f8e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1925
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

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