add nix flake for development shell

It does not support cross compilation but requires x86_64-linux.

nix/flake.nix

@@ -0,0 +1,35 @@
+{
+  outputs =
+    { self, nixpkgs }:
+    let
+      shell =
+        { pkgs }:
+        pkgs.mkShell {
+          packages = with pkgs; [
+            ncurses
+            dialog
+            perl
+            bashInteractive
+            flex
+            bison
+            buildPackages.qemu
+          ];
+          shellHook = ''
+            # Do not attempt cross compilation.
+            export CROSS_COMPILE=
+            # These are set by the compiler’s shellHook but mk refuses to work when these are set.
+            unset CC CPP CXX AR AS LD
+          '';
+        };
+
+      lib = import "${nixpkgs}/lib";
+      forAll = list: f: lib.genAttrs list f;
+    in
+    {
+      devShells = forAll [ "x86_64-linux" ] (system: {
+        default = shell {
+          pkgs = nixpkgs.legacyPackages.${system};
+        };
+      });
+    };
+}