fix: upload checksums.txt as artifact for formula updates

The update-formula job needs checksums.txt as an artifact to extract
bottle SHA256s. Previously, checksums.txt was only uploaded to the
GitHub release, causing the formula to be updated with empty SHA256s.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: timvw

.github/workflows/release.yml

@@ -166,6 +166,13 @@ jobs:
           shasum -a 256 * > checksums.txt
           cat checksums.txt
 
+      - name: Upload checksums artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: checksums
+          path: release/checksums.txt
+          if-no-files-found: error
+
       - name: Create GitHub Release
         env:
           GH_TOKEN: ${{ steps.generate-token.outputs.token }}