Create and use temp extraction directory in generate_appcast again

[zorgiepoo]

Fixes #2554

This restores how the code used to work in 2.6.0 except we don't create a link to the archive file, and remove the temp directory before starting. I had removed too much of the prior code in 2.6.1 which included creating the extraction directory in generate_appcast.

Misc Checklist

• [ ] My change requires a documentation update on Sparkle's website repository
• [x] 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
• [x] Other (please specify)

Tested generate_appcast on unarchiving:

• [x] DMG file
• [x] tar.* file
• [x] zip file

macOS version tested: 14.4.1 (23E224)