root: shell.nix for nixfmt

This uses the reusable pinned Nixpkgs from the parent commit to create a
shell.nix file for an environment with a pinned nixfmt version.

shell.nix

@@ -0,0 +1,30 @@
+# A shell to get tooling for Nixpkgs development
+#
+# Note: We intentionally don't use Flakes here,
+# because every time you change any file and do another `nix develop`,
+# it would create another copy of the entire ~500MB tree in the store.
+# See https://github.com/NixOS/nix/pull/6530 for the future
+{
+  system ? builtins.currentSystem,
+}:
+let
+  pinnedNixpkgs = builtins.fromJSON (builtins.readFile ci/pinned-nixpkgs.json);
+
+  nixpkgs = fetchTarball {
+    url = "https://github.com/NixOS/nixpkgs/archive/${pinnedNixpkgs.rev}.tar.gz";
+    sha256 = pinnedNixpkgs.sha256;
+  };
+
+  pkgs = import nixpkgs {
+    inherit system;
+    config = {};
+    overlays = [];
+  };
+in
+pkgs.mkShellNoCC {
+  packages = [
+    # The default formatter for Nix code
+    # https://github.com/NixOS/nixfmt
+    pkgs.nixfmt-rfc-style
+  ];
+}