cleanup: Always use single quotes

We have made good progress with transitioning to the new style,
to the point where we can complete it with a final push.

Start with changing the remaining places that still use double
quotes.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2866>

js/ui/main.js

@@ -337,7 +337,7 @@ async function _initializeUI() {
         LoginManager.registerSessionWithGDM();
 
         if (perfModule) {
-            let perfOutput = GLib.getenv("SHELL_PERF_OUTPUT");
+            let perfOutput = GLib.getenv('SHELL_PERF_OUTPUT');
             Scripting.runPerfScript(perfModule, perfOutput);
         }
     });