Build builder with 'build' instead of TryInto

[cmyr]

This is a little bit of cleanup to be more consistent in our (non)use of TryInto.

No functional change, JMM

[rsheeter]

This seems like a fairly reasonable use of TryInto, I'm not entirely sure why it needs to be fixed?

[rsheeter]

I don't find it obvious this is an improvement but I don't particularly object either.

[cmyr]

Yea, this is mostly about semantics and Rust idiom. I think this has come up before, but TryFrom is specifically about conversions between types that may fail in specific, controlled circumstances. The docs do a better job than I of explaining: http://doc.rust-lang.org/1.72.1/std/convert/trait.TryFrom.html