convert adobe-air.rb to use `installer :script`

* remove `caskroom_only true`
* remove `postflight`
convert amazon-music.rb to use `installer :script`

* remove `caskroom_only true`
* remove `postflight`
use `installer :script` in private-internet-access

* hoist primary command out of `postflight` into `installer` artifact
* this Cask did not use the `caskroom_only` idiom
Merge pull request #6869 from rolandwalker/installer_script_casks

use `installer :script` in 3 Casks