Fix missing libgssapi_krb5.so.2 in Flatpak builds

[limdingwen]

Fix missing libgssapi_krb5.so.2 in Flatpak builds

Description

Requires https://github.com/beeware/briefcase-linux-flatpak-template/pull/64 to be merged in first! In the meantime, you can use my template:

[tool.briefcase.app.kemonodownloader.linux.flatpak]
flatpak_runtime = "org.kde.Platform"
flatpak_runtime_version = "6.7"
flatpak_base = "com.riverbankcomputing.PyQt.BaseApp"
flatpak_base_version = "6.7"
flatpak_sdk = "org.kde.Sdk"
template = "https://github.com/limdingwen/briefcase-linux-flatpak-template/"

Before this commit, running briefcase build linux flatpak would work, but running it would cause a missing libgssapi_krb5.so.2 error and an instant crash.

One of the ways to fix this is by using https://github.com/flathub/com.riverbankcomputing.PyQt.BaseApp/ as the base, which this commit does.

Type of Change

• [X] Bug fix (non-breaking change that fixes an issue)
• [ ] New feature (non-breaking change that adds functionality)
• [ ] Breaking change (fix or feature that changes existing functionality)
• [ ] Documentation update
• [ ] Security fix
• [ ] Other (please describe):

How Has This Been Tested?

Tested on Linux, Fedora Silverblue. Flatpak building must be done on the host machine, not in a Toolbox. Python 3.13.7. briefcase build linux flatpak.

Checklist

• [X] My code follows the Contributing Guidelines and Code of Conduct.
• [X] My changes adhere to PEP 8 style guidelines.
• [X] I have tested my changes thoroughly using Briefcase on at least one supported platform.
• [X] I have updated the documentation (e.g., README.md, SUPPORT.md, in-app Help tab) if necessary.
• [X] My changes do not introduce new dependencies without prior discussion.
• [X] My changes respect the intellectual property rights and terms of service of Kemono.su and related platforms, as outlined in the README.
• [X] I have not included unverified links or promotional content.
• [X] For security-related changes, I have followed the Security Policy.

Additional Notes

Additional work needs to be done to publish a pre-built Flatpak.

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Note: Maintainers may request changes or additional information. Please respond promptly to feedback to ensure a smooth review process.

[limdingwen]

Converting to draft first until the dependent PR is merged.