The Drop impl for OutputType::BuiltinPager calls
handle.take().unwrap().join().unwrap() which panics if the
pager thread itself panicked. In Drop, this causes a double
panic (abort).
Replace with if-let and discard the join result, matching the
pattern used for OutputType::Pager which also uses let _ =.
Reported in #3449 where BAT_PAGER=builtin with --help
causes the pager thread to fail.
Fixes#3449
