cmd/go: quote expanded shell variables used within regular expressions

cmd/go: quote expanded shell variables used within regular expressions

We mostly use shell variables for paths, and we don't want file paths
like "C:\work\go1.4" to turn into regular expressions.

Updates #30228
Updates #30241

Change-Id: If18b775b2f8b2821eaf197c4be4a322066af839f
Reviewed-on: https://go-review.googlesource.com/c/164626
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>