nix: add shell starter

nix/shell.nix

@@ -0,0 +1,8 @@
+{system ? builtins.currentSystem}:
+let
+  pkgs = import <nixpkgs> {inherit system;};
+  artiq = pkgs.callPackage ./default.nix {};
+in
+pkgs.mkShell {
+  buildInputs = [ artiq ];
+}