fix(desktop): disable in-app updater for non-AppImage Linux installs

[jeevan6996]

Issue for this PR

Closes #23538

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

On Linux package-manager installs (RPM/DEB), Electron's in-app updater flow can restart without actually applying the update, which causes repeated "install and restart" prompts for the same version.

This gates desktop updater support on Linux to AppImage builds only (APPIMAGE present), while keeping existing updater behavior for packaged macOS/Windows builds.

How did you verify your code works?

From packages/desktop-electron:

• bun test src/main/updater-support.test.ts
• bun typecheck

(Pre-push also ran workspace turbo typecheck.)

Screenshots / recordings

Not a UI layout change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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