nix: refactor flake to avoid building jj for the dev shell

We don't actually need `jj` itself in the development shell. But having it in
the overlay significantly increases the time it takes to build the shell.

flake.nix

@@ -21,46 +21,39 @@
     in
     {
       overlays.default = (final: prev: {
-        jujutsu = final.callPackage
+        jujutsu = self.packages.${final.system}.jujutsu;
-          (
+      });
-            { stdenv
+    } //
-            , lib
+    (foreachSystem (system:
-            , fetchFromGitHub
+      let
-            , rustPlatform
+        pkgs = import nixpkgs {
-            , pkg-config
+          inherit system;
-            , openssl
+          overlays = [
-            , dbus
+            rust-overlay.overlays.default
-            , sqlite
+          ];
-            , file
+        };
-            , gzip
+      in
-            , makeWrapper
+      {
-            , Security
+        packages.${system} = {
-            , SystemConfiguration
+          jujutsu = pkgs.rustPlatform.buildRustPackage rec {
-            , libiconv
-            , installShellFiles
-            }:
-
-            rustPlatform.buildRustPackage rec {
             pname = "jujutsu";
             version = "unstable-${self.shortRev or "dirty"}";
             buildNoDefaultFeatures = true;
             buildFeatures = [ "jujutsu-lib/legacy-thrift" ];
-
+            src = ./.;
-              src = self;
-
             cargoLock = {
-                lockFile = "${self}/Cargo.lock";
+              lockFile = ./Cargo.lock;
             };
-              nativeBuildInputs = [
+            nativeBuildInputs = with pkgs; [
               gzip
               installShellFiles
               makeWrapper
               pkg-config
             ];
-              buildInputs = [ openssl dbus sqlite ]
+            buildInputs = with pkgs; [ openssl dbus sqlite ]
             ++ lib.optionals stdenv.isDarwin [
-                Security
+              darwin.apple_sdk.frameworks.Security
-                SystemConfiguration
+              darwin.apple_sdk.frameworks.SystemConfiguration
               libiconv
             ];
             postInstall = ''
@@ -74,33 +67,14 @@
               $out/bin/jj debug completion --zsh > ./completions.zsh
               installShellCompletion --zsh --name _${pname} ./completions.zsh
             '';
-            }
-          )
-          {
-            inherit (final.darwin.apple_sdk.frameworks) Security SystemConfiguration;
           };
-      });
-    } //
-    (foreachSystem (system:
-      let
-        pkgs = import nixpkgs {
-          inherit system;
-          overlays = [
-            self.overlays.default
-            rust-overlay.overlays.default
-          ];
-        };
-      in
-      {
-        packages.${system} = {
-          jujutsu = pkgs.jujutsu;
           default = self.packages.${system}.jujutsu;
         };
         apps.${system}.default = {
           type = "app";
-          program = "${pkgs.jujutsu}/bin/jj";
+          program = "${self.packages.${system}.jujutsu}/bin/jj";
         };
-        checks.${system}.jujutsu = pkgs.jujutsu.overrideAttrs ({ ... }: {
+        checks.${system}.jujutsu = self.packages.${system}.jujutsu.overrideAttrs ({ ... }: {
           cargoBuildType = "debug";
           cargoCheckType = "debug";
           preCheck = ''