proctoring.js isCameraAllowed definition causes redeclaration error with cachejs enabled

[35grain]

Prior to 062b530, the variable decalaration in proctoring.js was:

var isCameraAllowed = false;

In newer versions, let is used:

let isCameraAllowed = false;

This, however, causes the following error on our site when the cachejs setting in Moodle is turned on:

Uncaught SyntaxError: Indentifier 'isCameraAllowed' has already been declared.

As far as I can tell, the console error appears in all (authenticated) views.