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 <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Bryan Mills <[email protected]>
Reviewed-by: Than McIntosh <[email protected]>

Author: qmuntal

src/cmd/go/internal/script/cmds.go

@@ -188,7 +188,7 @@ func Cmp() Cmd {
 			},
 		},
 		func(s *State, args ...string) (WaitFunc, error) {
-			return nil, doCompare(s, true, args...)
+			return nil, doCompare(s, false, args...)
 		})
 }