Use system_command instead of system in Casks

[jawshooah]

system_command was introduced way back in #5723 as an interface for Casks to run shell commands in *flight blocks without using backticks or Kernel#system, allowing us to capture the output and error out if the command failed.

We've had some issues in the past where Casks silently failed to install or uninstall because a system call in a *flight block failed but no error was raised. Using system_command, such failures will be made apparent to the user, and thus to us when users create bug reports.

We should also create a cop over in rubocop-cask to check for instances of system and warn the user to replace them with system_command. We could even make it autocorrect to further reduce the burden on the user.

---

Action items:

• Replace existing instances of system in Casks with system_command
• Create a cop to warn when a Cask uses system instead of system_command
  • Teach the cop to autocorrect

[tsparber]

Would it also be possible to just "overwrite" the Kernel#system command in a similar way as homebrew does it?

[jawshooah]

This is their overridden system method that's exposed to Formula authors; we could potentially do something similar by exposing a system method on Hbc::DSL::Base.

[miccal]

First Action item is complete - all instances of system have been replaced with system_command in all repositories.

[vitorgalvao]

Closing in favour of https://github.com/caskroom/rubocop-cask/issues/46.