jll build in github cache

```
julia-cache;workflow=Build Reactant_jll;job=define-matrix;os=Linux;run_id=30143068145;run_attempt=1
```

is in https://github.com/EnzymeAD/Enzyme-JAX/actions/caches

but we shouldn't build that on the gh runners, I thought, @giordano ?

Start by locating the GitHub Actions workflow and the `Build Reactant_jll` `define-matrix` job, then inspect the referenced cache entry and its workflow configuration. Confirm which step creates the Julia cache on GitHub-hosted runners; done means that cache is no longer produced there while the intended build still works.

Written by the indexing model from the issue text.