refactor(bazel): use configured Rust toolchain root

configs/nixos/modules/shell/rust.nix

@@ -8,18 +8,28 @@
 let
   cfg = config.modules.shell.rust;
   user = config.user.name;
+  toolchain = pkgs.symlinkJoin {
+    name = "rust-toolchain";
+    paths = with pkgs; [
+      cargo
+      clippy
+      gcc
+      rustc
+      rustfmt
+    ];
+  };
 in
 {
   options.modules.shell.rust.enable = lib.mkEnableOption "Rust tools for the primary user's shell environment";
 
   config = lib.mkIf cfg.enable {
-    home-manager.users.${user}.home.packages = with pkgs; [
-      cargo
-      clippy
-      gcc
-      rust-analyzer
-      rustc
-      rustfmt
-    ];
+    home-manager.users.${user} = {
+      home.packages = [
+        toolchain
+        pkgs.rust-analyzer
+      ];
+
+      home.sessionVariables.RUST_TOOLCHAIN_ROOT = "${toolchain}";
+    };
   };
 }