e2e.yaml: Avoid non-versioned TUF metadata

Stop downloading non-versioned TUF metadata in the e2e workflow

I'd like to stop publishing non-versioned TUF metadata in the staging repository (and later in production): real clients should not be using non-versioned metadata anyway.
I chose 4.root.json just because the content matches the deprecated versioned URL
Added user agent to help future debugging

sigstore / gitsign