Building AppImages on macOS is No Longer Supported on v0.3.15

[rmartin16]

Describe the bug

When #1093 introduced Host OS checking for tools, it limited linuxdeploy to only running on Linux.

Therefore, while the briefcase create linux system command will complete on macOS, briefcase build linux appimage errors with linuxdeploy is not supported on Darwin; previously, it was possible to build and package an AppImage on macOS.

Steps to reproduce

Run briefcase build linux appimage on macOS using Briefcase v0.3.15.

Expected behavior

AppImages can be built and distributed from macOS using Docker.

Screenshots

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Environment

• Operating System: monterey
• Python version: 3.11.3
• Software versions:
  • Briefcase: 0.3.15

Logs

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Additional context

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