fix(caveats): avoid showing Rosetta 2 caveat on Intel machines

[taronsung]

Summary

When a cask uses the requires_rosetta built-in caveat, mark on_system_blocks_exist on the DSL so that architecture variations are properly computed during API JSON generation.

Problem

Intel Mac users see the Rosetta 2 caveat when installing casks via the API, even though Rosetta 2 is only needed on Apple Silicon Macs. This happens because:

1. The API JSON is generated while simulating ARM architecture
2. The requires_rosetta caveat includes the Rosetta warning in the base JSON
3. Variations for Intel architecture are NOT computed because on_system_blocks_exist? is false (the requires_rosetta caveat didn't set it)
4. When an Intel user installs the cask, they get the pre-computed ARM caveats without the Intel variation override

Solution

Call @cask.mark_on_system_blocks_exist! in the requires_rosetta caveat to ensure architecture variations are computed. This means:

• ARM: Rosetta caveat is included (correct)
• Intel variation: Rosetta caveat is nil (correct - will override the base)

When the cask is loaded on an Intel machine via the API, merge_variations will apply the Intel variation which has no Rosetta caveat.

Test

All changed-file tests pass with brew tests --changed.

Fixes #18365

[MikeMcQuaid]

@taronsung did you use AI to perform any part of this PR? If so, which one, for what parts and what was your prompt(s)?