cmd/go: replace backslash systematically in path of command

Using the same method CleanPatterns harmonizes further accepted format of patterns in go command.

Fixes #24233

Change-Id: Idb8176df3a7949b16764cd6ea51d7a8966799e42
Reviewed-on: https://go-review.googlesource.com/c/go/+/669775


LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>