go/build/constraint: use strings.Builder instead of for { str+=str }

go/build/constraint: use strings.Builder instead of for { str+=str }

(This works around a bug in the stringsbuilder modernizer.)

For #76476

Change-Id: I1cb8715fd79c0363cb9c159686eaeb3482c93228
Reviewed-on: https://go-review.googlesource.com/c/go/+/724721
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Bypass: Alan Donovan <adonovan@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>