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 ];
+  };
+}