perf: Use var for METRICS

Each perf module exposes it to the scripting module to report metrics
after all tests have been run. The symbol's scope should allow that,
so declare it as var.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/440

js/perf/hwtest.js

@@ -2,7 +2,7 @@ const { Clutter, Gio, Gtk, Shell } = imports.gi;
 const Main = imports.ui.main;
 const Scripting = imports.ui.scripting;
 
-let METRICS = {
+var METRICS = {
     timeToDesktop:
     { description: "Time from starting graphical.target to desktop showing",
       units: "us" },