internal/version: add comment about envutil.Dedup

So somebody doesn't accidentally delete it too soon.

Debian stable (stretch) has Go 1.7, for instance, where envutil.Dedup
still matters.

Change-Id: Ic6230e924cd9463b12a13b60fc01cd8deee63774
Reviewed-on: https://go-review.googlesource.com/c/144698
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
diff --git a/internal/version/version.go b/internal/version/version.go
index efcc257..3704b39 100644
--- a/internal/version/version.go
+++ b/internal/version/version.go
@@ -62,6 +62,9 @@
 	if p := os.Getenv("PATH"); p != "" {
 		newPath += string(filepath.ListSeparator) + p
 	}
+	// This envutil.Dedup call is unnecessary when the binary is
+	// built with Go 1.9+, but keep it around for now until Go 1.8
+	// is no longer seen in the wild in common distros.
 	cmd.Env = envutil.Dedup(caseInsensitiveEnv, append(os.Environ(), "GOROOT="+root, "PATH="+newPath))
 	if err := cmd.Run(); err != nil {
 		// TODO: return the same exit status maybe.