cmd/go/internal/script: Cmp() should not perform environment substitutions

cmd/go/internal/script: Cmp() should not perform environment substitutions

This is an oversight from https://go-review.googlesource.com/c/go/+/419875,
where script commands were refactored and factored out to a new package.

For #27494.

Change-Id: Ie606cab39f60859ee1da5165dcc94c8470c94325
Reviewed-on: https://go-review.googlesource.com/c/go/+/447575
Run-TryBot: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>