ci: update extra_nix_config to fix warnings in GitHub Actions

Author: nau

.github/workflows/ci.yml

@@ -20,11 +20,14 @@ jobs:
         with:
           nix_path: nixpkgs=channel:nixos-unstable
           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://cache.iog.io https://cache.nixos.org/
+            trusted-public-keys = hydra.iohk.io:f/Ea+s+dFdN+3Y/G+FDgSq+a5NEWhJGzdjvKNGv0/EQ= cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= iohk.cachix.org-1:DpRUyj7h7V830dp/i6Nti+NEO2/nhblbov/8MW7Rqoo= nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs=
+            substituters = https://cache.iog.io https://cache.nixos.org/ https://iohk.cachix.org https://nix-community.cachix.org
+            accept-flake-config = true
+            allow-import-from-derivation = true
+            experimental-features = nix-command flakes
       - uses: DeterminateSystems/magic-nix-cache-action@main
       - name: Build and test in Nix devshell
-        run: nix develop .#ci --command bash -c "sbt ci"
+        run: nix develop .#ci --accept-flake-config --command bash -c "sbt ci"
       - name: Report test results
         uses: sarisia/actions-status-discord@v1
         if: always()