update autodiff install docs for nix

src/autodiff/installation.md

@@ -18,30 +18,30 @@ Please run:
 rustup +nightly component add enzyme
 ```
 
-## Installation guide for Nix user.
+## Installation guide for Nix
+
+On [Nix](https://nixos.org/), you can declare a nightly Rust toolchain with the Enzyme component using the [oxalica rust-overlay](https://github.com/oxalica/rust-overlay).
+
+For example:
+
+```nix
+rust-bin.selectLatestNightlyWith (toolchain: toolchain.default.override {
+  extensions = [ "enzyme" ];
+})
+```
+
+Alternativley, you can create a [toolchain file](https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file) that declares the Enzyme component such as
+
+```toml
+[toolchain]
+channel = "nightly-2026-06-23"
+components = [ "enzyme" ]
+```
+
+and consume it in the overlay
 
-This setup was recommended by a nix and autodiff user.
-It uses [`Overlay`].
-Please verify for yourself if you are comfortable using that repository.
-In that case you might use the following nix configuration to get a rustc that supports `std::autodiff`.
 ```nix
-{
-  enzymeLib = pkgs.fetchzip {
-    url = "https://ci-artifacts.rust-lang.org/rustc-builds/ec818fda361ca216eb186f5cf45131bd9c776bb4/enzyme-nightly-x86_64-unknown-linux-gnu.tar.xz";
-    sha256 = "sha256-Rnrop44vzS+qmYNaRoMNNMFyAc3YsMnwdNGYMXpZ5VY=";
-  };
-
-  rustToolchain = pkgs.symlinkJoin {
-    name = "rust-with-enzyme";
-    paths = [pkgs.rust-bin.nightly.latest.default];
-    nativeBuildInputs = [pkgs.makeWrapper];
-    postBuild = ''
-      libdir=$out/lib/rustlib/x86_64-unknown-linux-gnu/lib
-      cp ${enzymeLib}/enzyme-preview/lib/rustlib/x86_64-unknown-linux-gnu/lib/libEnzyme-22.so $libdir/
-      wrapProgram $out/bin/rustc --add-flags "--sysroot $out"
-    '';
-  };
-}
+rust-bin.fromRustupToolchainFile ./rust-toolchain.toml
 ```
 
 ## Build instructions