Relative URL support for appcasts was added in #1617, but relativeToURL was not passed down by the constructor used to create delta appcast items, meaning when the updater tried to download a delta from a relative path, it resulted in the following error being logged:

Failed to download delta update. Falling back to regular update...
Error: An error occurred while downloading the update. Please try again later. (null) (URL assets/<snip>.delta)
Error: unsupported URL (null) (URL assets/<snip>.delta)

This PR fixes it so that deltas can also have relative URLs.

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)

See unit test update. Pipeline without the change here to show that the problem did exist, and was resolved by this change.

macOS version tested: 26.0 and 16.4