chore(ci): fix upload

[JP-Ellis]

Fix an issue resulting in no artifacts being uploaded to the release. The issue is two-fold:

• The download-artifact action by default, when there are multiple sources, is to extract each artifact in its own subdirectory.
• The upload-release-action only matches for files in the rust/release_artifacts directory, and importantly not any subdirectory.

The fix here is to simply toggle the merge-multiple to prevent artifacts from being extracted into separate subdirectories.