Fix wasmtime's caching with local development builds

Thought we had an issue tracking this but I guess not.

Cache entries that are stale due to local changes that have been made since the cache entry was created are reused nonetheless due to how we just check Wasmtime version (by default? I guess we have some custom version serialization stuff now, not sure how relevant it is to this)

One thing we could maybe do would be to add the hash of the whole executable to the cache key. This would be a big hammer, but would be pretty robust against any kind of local dirty state, or new commits on the same branch, or anything like that.