Create tar.xz files with built-in tar and remove bzip2 fallback

[zorgiepoo]

This removes usages of non-built-in xz utility when it was not needed for creating a Sparkle binary distribution. This also removes the fall back to bzip2 and always create .tar.xz distribution.

Misc Checklist

• My change requires a documentation update on Sparkle's website repository
• My change requires changes to generate_appcast, generate_keys, or sign_update

Testing

I tested and verified my change by using one or multiple of these methods:

• Sparkle Test App
• Unit Tests
• My own app
• Other (please specify)

Tested make release outputted identical binaries as before these changes (only dSYM files differed). make-release-package.sh tests code signing integrity of extracted products are valid as well. The tar.xz file created is a bit bigger than before (few MB more) but this shouldn't really matter much in practice.

macOS version tested: 14.4.1 (23E224)