ProgressBar::ask: Accept EOF as a no

This may occur when stderr is a tty but stdin is empty.
E.g.

    $ nix build </dev/null
    error: unexpected EOF reading a line

These stdio handles are how some non-interactive sandboxes behave,
including the Nix build sandbox and Hercules CI Effects.

src/libmain/progress-bar.cc

@@ -543,7 +543,7 @@ public:
         auto state(state_.lock());
         if (!state->active) return {};
         std::cerr << fmt("\r\e[K%s ", msg);
-        auto s = trim(readLine(STDIN_FILENO));
+        auto s = trim(readLine(STDIN_FILENO, true));
         if (s.size() != 1) return {};
         draw(*state);
         return s[0];