Overzealous logging

While on the surface it looks to be [throttled to once per minute](https://github.com/Automattic/dserve/blob/master/src/builder.ts#L207), in practice we've sometimes seen the message `There are images waiting to be built that are stuck because of too many concurrent builds` tens of times per second.

Start at src/builder.ts around line 207, where the issue says the warning is intended to be throttled. Reproduce or trace the path that emits “There are images waiting to be built that are stuck because of too many concurrent builds,” then verify that repeated conditions no longer produce messages tens of times per second while the intended once-per-minute behavior remains.

Written by the indexing model from the issue text.