Add initial flake.nix; just for develop, for now (fixes#3577)

flake.nix

@@ -0,0 +1,40 @@
+{
+  description = "bat";
+
+  inputs.nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+
+  outputs =
+    { self, ... }@inputs:
+    let
+      supportedSystems = [
+        "x86_64-linux" # 64-bit Intel/AMD Linux
+        "aarch64-linux" # 64-bit ARM Linux
+        "aarch64-darwin" # 64-bit ARM macOS
+      ];
+
+      forEachSupportedSystem =
+        f:
+        inputs.nixpkgs.lib.genAttrs supportedSystems (
+          system:
+          f {
+            inherit system;
+            pkgs = import inputs.nixpkgs {
+              inherit system;
+              config.allowUnfree = true;
+            };
+          }
+        );
+    in
+    {
+      devShells = forEachSupportedSystem (
+        { pkgs, system }:
+        {
+          default = pkgs.mkShellNoCC {
+            packages = with pkgs; [
+              cargo
+            ];
+          };
+        }
+      );
+    };
+}