Add nix flake

flake.nix

@@ -0,0 +1,43 @@
+{
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs";
+    utils.url = "github:numtide/flake-utils";
+    naersk.url = "github:nix-community/naersk";
+  };
+
+  outputs = { self, nixpkgs, utils, naersk, fenix }: utils.lib.eachDefaultSystem
+    (system:
+      let
+        name = "rl";
+        version = "latest";
+        # https://discourse.nixos.org/t/using-nixpkgs-legacypackages-system-vs-import/17462/7
+        pkgs = nixpkgs.legacyPackages.${system};
+        naersk' = naersk.lib.${system};
+      in
+      with pkgs;
+      rec {
+        packages = {
+          default = packages.${name};
+          "${name}" = naersk'.buildPackage {
+            inherit name version;
+            src = ./.;
+          };
+        };
+
+        apps = {
+          default = apps.${name};
+          "${name}" = utils.lib.mkApp {
+            drv = packages.default;
+            exePath = "/bin/${name}";
+          };
+        };
+
+        devShell = mkShell {
+          packages = [
+            cargo-audit
+            mask
+          ];
+        };
+      }
+    );
+}