attempt to use cache in Nix Shell Github Action

https://github.com/cachix/install-nix-action#how-do-i-add-a-binary-cache

Author: peterbecich

.github/workflows/nix-shell.yml

@@ -16,4 +16,7 @@ jobs:
     - uses: cachix/install-nix-action@v16
       with:
         nix_path: nixpkgs=channel:nixos-21.11
+        extra_nix_config: |
+          trusted-public-keys = hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY=
+          substituters = https://hydra.iohk.io https://cache.nixos.org/
     - run: nix-shell --pure --run "cabal update && cabal build all --enable-tests"