@Solene note that not all compilers are deterministic (for instance GHC when -j n with n greater than 1).
If you are interested into using Hydra with CA derivations you con try this, it seems to work if you don’t use remote builders. Also any help to make it work with remote builders is welcome 
Also, CA derivations and Hydra could bring to some unpleasant situations where you build more stuff than without CA derivations: https://github.com/NixOS/rfcs/blob/8bb86f8bddd98deb3c03c5698d5eff0b9072d0a7/rfcs/0062-content-addressed-paths.md#caching-of-non-deterministic-paths