build: add Bazel Rust workflow

configs/nixos/modules/shell/bazel.nix

@@ -0,0 +1,18 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# Bazel for the primary user's shell environment.
+let
+  cfg = config.modules.shell.bazel;
+  user = config.user.name;
+in
+{
+  options.modules.shell.bazel.enable = lib.mkEnableOption "Bazel for the primary user's shell environment";
+
+  config = lib.mkIf cfg.enable {
+    home-manager.users.${user}.home.packages = [ pkgs.bazel ];
+  };
+}

configs/nixos/modules/shell/rust.nix

@@ -0,0 +1,35 @@
+{
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+# Rust tools for the primary user's shell environment.
+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.${user} = {
+      home.packages = [
+        toolchain
+        pkgs.rust-analyzer
+      ];
+
+      home.sessionVariables.RUST_TOOLCHAIN_ROOT = "${toolchain}";
+    };
+  };
+}