Engineers simply commit artifacts with Git LFS as a signed commit. Totally unprivileged build systems can append reproducible build signatures via git tags. That repo can be webhook triggered to be -pulled- by a lambda job or similar in the target environment that will the verify tags and signatures to assert if it is valid, and deploy artifacts to signature approved environments using ephemeral role credentials.

A VCS system or CI system should never have secrets or be trusted in any way. Doing this is always dramatically increases attack surface for no reason.

I run a security consulting firm and this is often one of the first things I help my clients to fix.

Testing is a separate concern than supply chain security. Testing should also never require any secrets useful to an adversary, so third party hosted CI is low risk here.