fix(fish): export Rust toolchain root

configs/nixos/modules/shell/fish/default.nix

@@ -9,7 +9,11 @@ let
   cfg = config.modules.shell.fish;
   user = config.user.name;
   shortcuts = config.registry.shell-shortcut.${config.networking.hostName};
+  envVars = config.registry.env-var.${config.networking.hostName};
   commandsOf = shortcuts: lib.mapAttrs (_name: shortcut: shortcut.command) shortcuts;
+  exportEnvVar = name: value: ''
+    set -gx ${name} ${lib.escapeShellArg value}
+  '';
 in
 {
   options.modules.shell.fish = {
@@ -99,7 +103,10 @@ in
         };
 
         # Rendered by home-manager into ~/.config/fish/config.fish.
-        interactiveShellInit = builtins.readFile ./config.fish;
+        interactiveShellInit = lib.concatStringsSep "\n" (
+          (lib.mapAttrsToList exportEnvVar envVars)
+          ++ [ (builtins.readFile ./config.fish) ]
+        );
       };
     };
   };

configs/nixos/modules/shell/rust.nix

@@ -23,13 +23,13 @@ in
   options.modules.shell.rust.enable = lib.mkEnableOption "Rust tools for the primary user's shell environment";
 
   config = lib.mkIf cfg.enable {
+    registry.env-var.${config.networking.hostName}.RUST_TOOLCHAIN_ROOT = "${toolchain}";
+
     home-manager.users.${user} = {
       home.packages = [
         toolchain
         pkgs.rust-analyzer
       ];
-
-      home.sessionVariables.RUST_TOOLCHAIN_ROOT = "${toolchain}";
     };
   };
 }

configs/nixos/registry/env-var.nix

@@ -0,0 +1,4 @@
+{ lib, ... }:
+{
+  type = lib.types.str;
+}