[patch] Fix Assist on 8.x logic

[durera]

We added code to stop presenting the option to install Assist in 8.x, but did not initialise installAssist = False, resulting in the following:

Traceback (most recent call last):
  File "/opt/app-root/bin/mas-install", line 964, in <module>
    app.install(args)
  File "/opt/app-root/bin/mas-install", line 881, in install
    self.interactiveMode()
  File "/opt/app-root/bin/mas-install", line 536, in interactiveMode
    self.assistSettings()
  File "/opt/app-root/bin/mas-install", line 497, in assistSettings
    if self.installAssist:
AttributeError: 'App' object has no attribute 'installAssist'