perf(rows): use strings builder types

[ayuhito]

Not very impactful, but why not. Each concat is a separate heap allocation that is discarded by GC, so strings.Builder is a little bit more hygienic for this use case especially since we are appending in an unknown loop.

[taniabogatsch]

Thanks! Reducing heap allocations is great - even though, like you said, this is probably not the most performance-critical code path.