Pin the rust version in flake.nix

Before we were just using whatever was the latest stable (according to
the time that our flake.lock file was updated. This was Rust 1.69.0 FYI.

Now we're specifically pinning the version of Rust. Synapse only
requires Rust 1.60.0.

flake.nix

@@ -46,20 +46,20 @@
     systems.url = "github:nix-systems/default";
     # A development environment manager built on Nix. See https://devenv.sh.
     devenv.url = "github:cachix/devenv/main";
-    # Rust toolchains and rust-analyzer nightly.
-    fenix = {
-      url = "github:nix-community/fenix";
-      inputs.nixpkgs.follows = "nixpkgs";
-    };
+    # Rust toolchain.
+    rust-overlay.url = "github:oxalica/rust-overlay";
   };
 
-  outputs = { self, nixpkgs, devenv, systems, ... } @ inputs:
+  outputs = { self, nixpkgs, devenv, systems, rust-overlay, ... } @ inputs:
     let
       forEachSystem = nixpkgs.lib.genAttrs (import systems);
     in {
       devShells = forEachSystem (system:
         let
-          pkgs = nixpkgs.legacyPackages.${system};
+          overlays = [ (import rust-overlay) ];
+          pkgs = import nixpkgs {
+            inherit system overlays;
+          };
         in {
           # Everything is configured via devenv - a Nix module for creating declarative
           # developer environments. See https://devenv.sh/reference/options/ for a list
@@ -76,6 +76,17 @@
                 # Configure packages to install.
                 # Search for package names at https://search.nixos.org/packages?channel=unstable
                 packages = with pkgs; [
+                  # The rust toolchain and related tools.
+                  # This will install the "default" profile of rust components.
+                  # https://rust-lang.github.io/rustup/concepts/profiles.html
+                  (rust-bin.stable."1.60.0".default.override {
+                    # Additionally install the "rust-src" extension to allow diving into the
+                    # Rust source code in an IDE (rust-analyzer will also make use of it).
+                    extensions = [ "rust-src" ];
+                  })
+                  # The rust-analyzer language server implementation.
+                  rust-analyzer
+
                   # Native dependencies for running Synapse.
                   icu
                   libffi
@@ -124,12 +135,11 @@
                 # Install dependencies for the additional programming languages
                 # involved with Synapse development.
                 #
-                # * Rust is used for developing and running Synapse.
                 # * Golang is needed to run the Complement test suite.
                 # * Perl is needed to run the SyTest test suite.
+                # * Rust is used for developing and running Synapse.
+                #   It is installed manually with `packages` above.
                 languages.go.enable = true;
-                languages.rust.enable = true;
-                languages.rust.version = "stable";
                 languages.perl.enable = true;
 
                 # Postgres is needed to run Synapse with postgres support and