Use fenix toolchain in nix shell (#18227)

In #17974 we explicitly depend on rustc/cargo for the nix devShell,
however the fenix overlay that contains the latest stable versions was
not being applied to that shell. This led to the shell inheriting
whatever rustc/cargo was on nixos-unstable from nixpkgs, which sometimes
lags behind. This change fixes that, and also restructures the flake to
ensure that all outputs rely on the overlaid `pkgs`.

Release Notes:

- N/A

flake.nix

@@ -17,27 +17,34 @@
     fenix,
     ...
   }: let
-    forAllSystems = function:
-      nixpkgs.lib.genAttrs [
-        "x86_64-linux"
-        "aarch64-linux"
-      ] (system:
-        function (import nixpkgs {
-          inherit system;
-          overlays = [fenix.overlays.default];
-        }));
+    systems = ["x86_64-linux" "aarch64-linux"];
+
+    overlays = {
+      fenix = fenix.overlays.default;
+      rust-toolchain = final: prev: {
+        rustToolchain = final.fenix.stable.toolchain;
+      };
+      zed-editor = final: prev: {
+        zed-editor = final.callPackage ./nix/build.nix {
+          craneLib = (crane.mkLib final).overrideToolchain final.rustToolchain;
+          rustPlatform = final.makeRustPlatform {
+            inherit (final.rustToolchain) cargo rustc;
+          };
+        };
+      };
+    };
+
+    mkPkgs = system:
+      import nixpkgs {
+        inherit system;
+        overlays = builtins.attrValues overlays;
+      };
+
+    forAllSystems = f: nixpkgs.lib.genAttrs systems (system: f (mkPkgs system));
   in {
-    packages = forAllSystems (pkgs: let
-      craneLib = (crane.mkLib pkgs).overrideToolchain (p: p.fenix.stable.toolchain);
-      rustPlatform = pkgs.makeRustPlatform {
-        inherit (pkgs.fenix.stable.toolchain) cargo rustc;
-      };
-      nightlyBuild = pkgs.callPackage ./nix/build.nix {
-        inherit craneLib rustPlatform;
-      };
-    in {
-      zed-editor = nightlyBuild;
-      default = nightlyBuild;
+    packages = forAllSystems (pkgs: {
+      zed-editor = pkgs.zed-editor;
+      default = pkgs.zed-editor;
     });
 
     devShells = forAllSystems (pkgs: {
@@ -46,13 +53,10 @@
 
     formatter = forAllSystems (pkgs: pkgs.alejandra);
 
-    overlays.default = final: prev: {
-      zed-editor = final.callPackage ./nix/build.nix {
-        craneLib = (crane.mkLib final).overrideToolchain (p: p.fenix.stable.toolchain);
-        rustPlatform = final.makeRustPlatform {
-          inherit (final.fenix.stable.toolchain) cargo rustc;
-        };
+    overlays =
+      overlays
+      // {
+        default = nixpkgs.lib.composeManyExtensions (builtins.attrValues overlays);
       };
-    };
   };
 }