In loadtest, make localAudio non-const. (#8829)

(Because we modify it.)

resources/load-test/load-test-participant.js

 const params = parseURLParams(window.location, false, 'hash');
 const { isHuman = false } = params;
 const {
-    localAudio = config.startWithAudioMuted !== true,
     localVideo = config.startWithVideoMuted !== true,
     remoteVideo = isHuman,
     remoteAudio = isHuman,
     autoCreateLocalAudio = config.testing.noAutoLocalAudio !== true
 } = params;
 
+let {
+    localAudio = config.startWithAudioMuted !== true,
+} = params;
+
 const { room: roomName } = parseURIString(window.location.toString());
 
 let connection = null;