flag: use strings.Builder instead of concatenating strings

There is a single function in the flag package whose implementation
uses string concatenation instead of the recommended strings.Builder.
The function was last touched before strings.Builder was introduced
in Go 1.10, which explains the old style code. This PR updates
the implementation.

Fixes #45392

Change-Id: Id2d8f1788765a0c4faaeb1e6870914f72b3c8442
GitHub-Last-Rev: 0e12fe304593afc627fc4f1597670efd354809b0
GitHub-Pull-Request: golang/go#45393
Reviewed-on: https://go-review.googlesource.com/c/go/+/307329


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Josh Bleecher Snyder <josharian@gmail.com>