cmd/internal/pkgpath: use strings.Builder

cmd/internal/pkgpath: use strings.Builder

Since when go1.17 is now used for bootstraping.

Change-Id: I5f763dec1cb152f94ab1c677d3fa26da17abf097
Reviewed-on: https://go-review.googlesource.com/c/go/+/427557
Reviewed-by: Heschi Kreinick <heschi@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Keith Randall <khr@golang.org>
Run-TryBot: hopehook <hopehook@golangcn.org>
Reviewed-by: Keith Randall <khr@google.com>