[DEVOPS-1090] Buildkite: Update hydra version in check-hydra.sh
[DEVOPS-1090] Buildkite: Update hydra version in check-hydra.sh

We are using a forked version of Hydra, but this version from nixpkgs
18.09 is probably closer.

input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl