Add flake package export

flake.nix

@@ -25,7 +25,18 @@
               rust-analyzer
               clippy
             ];
-            env = {};
+            env = { };
+          };
+
+          packages.wl-backdrop =
+            let
+              cargoToml = fromTOML (builtins.readFile ./Cargo.toml);
+            in
+            pkgs.rustPlatform.buildRustPackage {
+              inherit (cargoToml.package) name version;
+              src = ./.;
+              cargoLock.lockFile = ./Cargo.lock;
+              env = { };
             };
         };