go/build: skip string literal while findEmbed

The findEmbed function looking for comment by readbyte,
however it might have constant or variables that contains
comment.
Maybe we should use ast parser in the future.

Fixes #43373

Change-Id: I92544384fc4c11363d8b2f6b9898c8dea1602767
Reviewed-on: https://go-review.googlesource.com/c/go/+/280332


Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Meng Zhuo <mzh@golangcn.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>