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];

tests/functional/flakes/config.sh

@@ -27,7 +27,17 @@ cat <<EOF > flake.nix
 EOF
 
 # Without --accept-flake-config, the post hook should not run.
+# To test variations in stderr tty-ness, we run the command in different ways,
+# none of which should block on stdin or accept the `nixConfig`s.
 nix build < /dev/null
+nix build < /dev/null 2>&1 | cat
+# EOF counts as no, even when interactive (throw EOF error before)
+if type -p script >/dev/null && script -q -c true /dev/null; then
+    echo "script is available and GNU-like, so we can ensure a tty"
+    script -q -c 'nix build < /dev/null' /dev/null
+else
+    echo "script is not available or not GNU-like, so we skip testing with an added tty"
+fi
 (! [[ -f post-hook-ran ]])
 TODO_NixOS
 clearStore

tests/functional/package.nix

@@ -60,6 +60,7 @@ mkMesonDerivation (finalAttrs: {
     # etc.
     busybox-sandbox-shell
     # For Overlay FS tests need `mount`, `umount`, and `unshare`.
+    # For `script` command (ensuring a TTY)
     # TODO use `unixtools` to be precise over which executables instead?
     util-linux
   ];