ci: enable flakes

Author: CertainLach

.github/workflows/nix.yml

@@ -10,5 +10,8 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - uses: DeterminateSystems/nix-installer-action@main
+        with:
+          extra-conf: |-
+            extra-experimental-features = nix-command flakes impure-derivations ca-derivations
       - uses: DeterminateSystems/magic-nix-cache-action@main
       - run: nix flake check