libstore: optionally set the NIX_DEBUG environment variable

src/libstore/build/local-derivation-goal.cc

@@ -912,6 +912,16 @@ void LocalDerivationGoal::initEnv()
 
     /* Trigger colored output in various tools. */
     env["TERM"] = "xterm-256color";
+
+#ifdef NIX_DEBUG_ENVIRONMENT_VARIABLE
+    /*
+     * Allows controlling the NIX_DEBUG environment variable. In the Nixpkgs stdenv, when
+     * `NIX_DEBUG` is set to 1 or greater, more informative information is included in the logs.
+     *
+     * See https://nixos.org/manual/nixpkgs/stable/#var-stdenv-NIX_DEBUG
+     */
+    env["NIX_DEBUG"] = fmt("%d", NIX_DEBUG_ENVIRONMENT_VARIABLE);
+#endif
 }