chore: update assets.yaml for manual invocation

[alicejli]

...and another follow up to #1416

https://github.com/googleapis/gapic-showcase/actions/runs/7491935131/job/20394310060#step:8:1 shows:

Run svenstaro/upload-release-action@v2
  with:
    repo_token: ***
    file: ./dist/*
    overwrite: true
    file_glob: true
  env:
    TAG_NAME: v0.[3](https://github.com/googleapis/gapic-showcase/actions/runs/7491935131/job/20394310060#step:8:3)0.0
Error: Input required and not supplied: tag

This updates the workflow to use either the event tag or the manually supplied tag.