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
• 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 generate_appcast on unarchiving:

• DMG file
• tar.* file
• zip file

macOS version tested: 14.4.1 (23E224)