- Fixes apple#883.

Changes `BuildImageResolver` to use `fetch()` instead of `pull()`. 

`fetch()` checks the cache first and only pulls if needed, preventing
incorrect progress events (e.g., "1/1 KB" metadata checks) on cached
images that caused the progress bar width calculation to produce
negative/weird padding counts, triggering "Fatal error: Negative count
not allowed" crashes during rebuilds.