all: avoid string(i) where i has type int

all: avoid string(i) where i has type int

Instead use string(r) where r has type rune.

This is in preparation for a vet warning for string(i).

Updates #32479

Change-Id: Ic205269bba1bd41723950219ecfb67ce17a7aa79
Reviewed-on: https://go-review.googlesource.com/c/go/+/220844
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Akhil Indurti <aindurti@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Toshihiro Shiino <shiino.toshihiro@gmail.com>