screenshot: Replace template strings with .format()

CI will complain when we add screenshot.js to the POTFILES in the next
commit because gettext doesn't like template strings, so make CI happy
and switch to .format() instead.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1658>

js/ui/screenshot.js

@@ -90,11 +90,11 @@ var ScreenshotService = class {
             return null;
 
         yield Gio.File.new_for_path(
-            GLib.build_filenamev([path, `${filename}.png`]));
+            GLib.build_filenamev([path, '%s.png'.format(filename)]));
 
         for (let idx = 1; ; idx++) {
             yield Gio.File.new_for_path(
-                GLib.build_filenamev([path, `${filename}-${idx}.png`]));
+                GLib.build_filenamev([path, '%s-%s.png'.format(filename, idx)]));
         }
     }