User isolation works, the keychain isolation works. On macOS tokens stored in the keychain can be made readable only by specific apps, not anything else. It does require a bit of infrastructure - ideally a Mac app that does the release - but nothing you can't vibe code quickly.
That's true, but it seems far more complex than just moving trivy to a separate workerflow with no permissions and likely physical isolation between it and a credential. I'm pretty wary of the idea that malware couldn't just privesc - it's pretty trivial to obtain root on a user's laptop. Running as a separate, unprivileged user helps a ton, but again, I'm skeptical of this vs just using a github workflow.