nix-shell: pin nixpkgs commit for determinism (thus sanity)
nix-shell: pin nixpkgs commit for determinism (thus sanity)

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