iterm2: convert to on_system blocks

[viking1304]

Important: Do not tick a checkbox if you haven’t performed its action. Honesty is indispensable for a smooth review process.

In the following questions <cask> is the token of the cask you're submitting.

After making all changes to a cask, verify:

• The submission is for a stable version or documented exception.
• brew audit --cask --online <cask> is error-free.
• brew style --fix <cask> reports no offenses.

Additionally, if adding a new cask:

• Named the cask according to the token reference.
• Checked the cask was not already refused.
• Checked the cask is submitted to the correct repo.
• brew audit --new-cask <cask> worked successfully.
• brew install --cask <cask> worked successfully.
• brew uninstall --cask <cask> worked successfully.

[viking1304]

@bevanjkay Thanks! I will use the same live-check approach as you did here next time.

[BrewTestBot]

🤖 A scheduled task has triggered a merge.

[BrewTestBot]

⚠️ Publish failed.

merge failed

[bevanjkay]

/rebase