gimp@dev: add Linux AppImage

[AlternateRT]

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After making any changes to a cask, existing or new, 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 (add your cask's name to the end of the search field).
• brew audit --cask --new <cask> worked successfully.
• HOMEBREW_NO_INSTALL_FROM_API=1 brew install --cask <cask> worked successfully.
• brew uninstall --cask <cask> worked successfully.

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• I did not use AI/LLM to create this PR, or I disclosed the tool/model below and reviewed its output, including zap stanza paths; I did not attribute commits to AI and will answer maintainer questions and review comments myself without AI/LLM.

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[AlternateRT]

Had to force CI to run again because in the previous round, it incorrectly determined that the intel artifact for macOS was unsigned: https://github.com/Homebrew/homebrew-cask/actions/runs/31801680731/job/94771081600?pr=281073

I also ran gktool scan gimp-3.2.0-RC3-x86_64.dmg on my machine and got this result:

Scan completed, and would be allowed but the user still needs to approve it on first launch.