refactor(nixos): use Home Manager session variables

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

@@ -9,10 +9,6 @@ let
   cfg = config.modules.shell.fish; # NOLINT(let_order)
   commandsOf = shortcuts: lib.mapAttrs (_name: shortcut: shortcut.command) shortcuts;
   shortcuts = lib.attrByPath [ config.networking.hostName ] { } config.registry.shell-shortcut;
-  exportEnvVar = name: value: ''
-    set -gx ${name} ${lib.escapeShellArg value}
-  '';
-  envVars = lib.attrByPath [ config.networking.hostName ] { } config.registry.env-var;
   user = config.user.name;
 in
 {
@@ -106,9 +102,7 @@ in
         };
 
         # Rendered by home-manager into ~/.config/fish/config.fish.
-        interactiveShellInit = lib.concatStringsSep "\n" (
-          (lib.mapAttrsToList exportEnvVar envVars) ++ [ (builtins.readFile ./config.fish) ]
-        );
+        interactiveShellInit = builtins.readFile ./config.fish;
       };
     };
   };

configs/nixos/modules/shell/python.nix

@@ -14,8 +14,9 @@ in
     lib.mkEnableOption "Python tools for the primary user's shell environment";
 
   config = lib.mkIf cfg.enable {
-    registry.env-var.${config.networking.hostName}.PYTHON_TOOLCHAIN_ROOT = "${pkgs.python3}";
-
-    home-manager.users.${user}.home.packages = [ pkgs.python3 ];
+    home-manager.users.${user}.home = {
+      sessionVariables.PYTHON_TOOLCHAIN_ROOT = "${pkgs.python3}";
+      packages = [ pkgs.python3 ];
+    };
   };
 }

configs/nixos/modules/shell/rust.nix

@@ -24,9 +24,9 @@ in
     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.${user} = {
+      home.sessionVariables.RUST_TOOLCHAIN_ROOT = "${toolchain}";
+
       home.packages = [
         toolchain
         pkgs.rust-analyzer