Investigate --builder.urls usage

[wemeetagain]

Builder is a bit of an outlier right now, we allow to set multiple URLs via --builder.urls but only use the first URL (opts.urls[0]). I opted to only log the URL which is actually used.

@g11tech Is it intentional that we don't allow a fallback for the builder? I guess mev-boost is already doing the multiplexing and builder flow is stateful which makes fallback logic a bit problematic.

Maybe we want to reflect that in the CLI as well as currently it looks like it supports multiple urls

Originally posted by @nflaig in #6175 (comment)

[g11tech]

Right because of the multiplexing by mev boost , may be we can still multiplex on top but we haven't felt the need so far