Fix CI (#40)

.github/workflows/nix.yml

@@ -7,8 +7,11 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - uses: cachix/install-nix-action@v12
-    - uses: numtide/cachix-action@only-cache
+      with:
+        nix_path: nixpkgs=channel:nixos-unstable
+    - uses: cachix/cachix-action@v8
       with:
         name: nix-community
-        signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'
+        extraPullNames: nixpkgs-update
+        skipPush: true
     - run: nix-build