Guard update timer update check against sessionInProgress

[zorgiepoo]

Make sure we don't perform a background update check when a user update check was initiated. This may have regressed from #2295.

Also replaces an assert with an error log if the _driver already exists.

Fixes #2560

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 scheduled update checks still works with test app and user can still check for updates.

macOS version tested: 14.4.1 (23E224)