lei_query: fix relative path handling on --stdin
Since --stdin could be waiting on user keyboard input or
something else slow, we handle it in the event loop. That
means other commands can change the working directory of
lei-daemon while a query is being trickled to us via stdin.
Rearranging query handling internals to delay opening the
--output destination in commit
26e0fe73de93f451 meant
another command could throw off our --output pathname if
it is relative.
Fixes: 26e0fe73de93f451 ("lei_query: rearrange internals to capture query early")