feat: Add AppImage support for Linux Casksfeat: add appimage support for linux

[biplavbarua]

Description

This PR adds support for appimage artifacts in Casks, primarily for Linux support.

Changes

• Added Cask::Artifact::AppImage class.
• Added appimagedir configuration (defaults to ~/Applications).
• Added tests for AppImage artifact.
• Fixed test helper to include appimagedir.
• Overridden dsl_key to ensure appimage stanza works correctly.

Testing

• brew tests --only=cask/artifact/appimage passes.- [ ] Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew lgtm (style, typechecking and tests) with your changes locally?

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[SMillerDev]

If these are just my changes this will break cask CI because I had only half an attempt to make Linux only and partial arch requirement installs work.

[biplavbarua]

Hi! Thanks for the heads up.

I've actually built upon your initial work and fixed the issues to get the CI passing.

Specifically, I:

1. Added the missing dsl_key override so AppImages are correctly detected.
2. Fixed the Sorbet/Typecheck errors (including the strict instance_variable_get issues in the installer).
3. Added a new test suite (appimage_spec.rb).

The CI checks are currently all green ✅.

[SMillerDev]

None of those are the issue. The issue is that since there are essentially no Appimages that support ARM, the cask CI needs to be able to support selective architecture selections.

That part is currently not working, and needs more testing and coding. It should also probably be split out from this PR, since it's blocking a flutter PR in the cask repo.

[SMillerDev]

The problem isn't that appimages need special handling though. They problem is that if you define an architecture requirement in an on_linux block, the CI matrix builder does not honor it.