Communicate the structured log FD to builders

Since we may use a dedicated file descriptor in the future, this
allows us to change it. So builders can do

  if [[ -n $NIX_LOG_FD ]]; then
    echo "@nix { message... }" >&$NIX_LOG_FD
  fi

src/libstore/build.cc

@@ -2202,7 +2202,7 @@ void DerivationGoal::initEnv()
    env["NIX_BUILD_CORES"] = (format("%d") % settings.buildCores).str();

    /* In non-structured mode, add all bindings specified in the
       derivation via the environments, except those listed in the
       derivation via the environment, except those listed in the
       passAsFile attribute. Those are passed as file names pointing
       to temporary files containing the contents. Note that
       passAsFile is ignored in structure mode because it's not
@@ -2258,6 +2258,11 @@ void DerivationGoal::initEnv()
        Strings varNames = tokenizeString<Strings>(get(drv->env, "impureEnvVars"));
        for (auto & i : varNames) env[i] = getEnv(i);
    }

    /* Currently structured log messages piggyback on stderr, but we
       may change that in the future. So tell the builder which file
       descriptor to use for that. */
    env["NIX_LOG_FD"] = "2";
}