Fix flake definition and GitHub workflow

.github/workflows/gh-pages.yml

@@ -7,7 +7,7 @@ on:
 
 jobs:
   deploy:
-    runs-on: latest-ubuntu
+    runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
       - uses: cachix/install-nix-action@v16

flake.nix

@@ -12,6 +12,6 @@
       let pkgs = import nixpkgs { inherit system; };
     in {
       devShell =
-        import ./nix/shell.nix { inherit pkgs; };
+        import ./shell.nix { inherit pkgs; };
     });
 }