libutil: read window size from stdout if stderr fails

This fixes the output of `nix help 2>/dev/null` so it stops wrapping at
60 columns.

Change-Id: Id0000000842b5befd73afbd6aff2825bdeae46af

lix/libutil/terminal.cc

@@ -106,7 +106,7 @@ static Sync<std::pair<unsigned short, unsigned short>> windowSize{{0, 0}};
 void updateWindowSize()
 {
     struct winsize ws;
-    if (ioctl(2, TIOCGWINSZ, &ws) == 0) {
+    if (ioctl(2, TIOCGWINSZ, &ws) == 0 || ioctl(1, TIOCGWINSZ, &ws) == 0) {
         auto windowSize_(windowSize.lock());
         windowSize_->first = ws.ws_row;
         windowSize_->second = ws.ws_col;