nix flake: make the binary cache "just work" (#2999)

book/src/install.md

@@ -22,8 +22,12 @@ the project root. The flake can also be used to spin up a reproducible developme
 shell for working on Helix with `nix develop`.
 
 Flake outputs are cached for each push to master using
-[Cachix](https://www.cachix.org/). With Cachix
-[installed](https://docs.cachix.org/installation), `cachix use helix` will
+[Cachix](https://www.cachix.org/). The flake is configured to
+automatically make use of this cache assuming the user accepts
+the new settings on first use.
+
+If you are using a version of Nix without flakes enabled you can
+[install Cachix cli](https://docs.cachix.org/installation); `cachix use helix` will
 configure Nix to use cached outputs when possible.
 
 ### Arch Linux

flake.nix

@@ -96,4 +96,9 @@
         };
       };
     };
+
+  nixConfig = {
+    extra-substituters = ["https://helix.cachix.org"];
+    extra-trusted-public-keys = ["helix.cachix.org-1:ejp9KQpR1FBI2onstMQ34yogDm4OgU2ru6lIwPvuCVs="];
+  };
 }