Until we do so, every single form of input should be considered hostile. We've already seen LLMs run base64-encoded instructions[0], so even something as trivial as passing a list of commit shorthashes could be dangerous: someone could've encoded instructions in that, after all.
And all of that is before considering the possibility of a LLM going "rogue" and hallucinating needing to take actions it wasn't explicitly instructed to. I genuinely can't understand how people even for a second think it is a good idea to give a LLM access to production systems...
There's basically no reason for GitHub workflows to ever have any credentials by default; credentials should always be explicitly provisioned, and limited only to events that can be provenanced back to privileged actors (read: maintainers and similar). But GitHub Actions instead has this weird concept of "default-branch originated" events (like pull_request_target and issue_comment) that are significantly more privileged than they should be.
The LLM is a cute way to carry out this vulnerability, but in fact it's very easy to get code execution and poison a cache without LLMs, for example when executing code in the context of a unit test.
It is genuinely a wonder that we haven't seen massive supply-chain compromises yet. Imagine what kind of horror you could do by compromising "actions/cache" and using CD credentials to pivot to everyone's AWS / GCP / Azure environments!
You can also do a lot more with GHA compared to waiting for Zapier to support your scenario. Plus most people I knew who used Zapier connected it to some Lambda or another webhook where they got the data from there and ran arbitrary code anyway.