cmd/go: add two more uses of lazyregexp

A couple of regexp.MustCompile globals have made their way in since we
introduced this package for cmd/go.

Convert the declarations. It's just two of them, so the cmd/go exec
benchmark isn't affected noticeably.

Change-Id: Ibd0615c99b6a049124a952c59978fd714c1b9615
Reviewed-on: https://go-review.googlesource.com/c/go/+/179358


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>