krita: add linux appimage support

[SMillerDev]

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

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Accompanies Homebrew/brew#20334 as an example

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[Copilot]

Pull Request Overview

This PR adds Linux AppImage support to the Krita cask, enabling cross-platform installation through Homebrew. The changes introduce conditional logic to handle different download formats and installation methods for macOS and Linux platforms.

• Implements platform-specific download URLs using conditional suffixes
• Updates SHA256 checksums to support multiple architectures including Linux x86_64
• Adds Linux-specific installation logic with AppImage support

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[Copilot]

The ARM and x86_64 macOS checksums are identical, which is unlikely to be correct since these would be different binaries for different architectures.

Suggested change

	sha256 arm: "17ccfbf2b72fcca314ce59c6d6f0b62dda1dc230ece71aa42f56031ef75ea6e8",
	x86_64: "17ccfbf2b72fcca314ce59c6d6f0b62dda1dc230ece71aa42f56031ef75ea6e8",
	sha256 arm: "REPLACE_WITH_ACTUAL_ARM_SHA256",
	x86_64: "REPLACE_WITH_ACTUAL_X86_64_SHA256",

[github-actions]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[p-linnane]

We can reopen this when the Homebrew/brew PR is ready.