XapHelper.pm: fix QP_FLAGS initialization

XapHelper.pm: fix QP_FLAGS initialization

We can't use $PublicInbox::Search::QP_FLAGS until after
load_xapian is called.  Failure to set the correct query parser
flags was causing failures in
`PI_NO_CXX=1 TEST_DAEMON_XH=-X0 prove -bw t/imapd.t'
since phrase parsing was broken with the Perl bindings
XapHelper implementation.

Now, the `check-xh0' and `check-xh1' targets both pass with
PI_NO_CXX=1 set to disable the C++ xap_helper.

Fixes: fa6a7919 (xap_helper: enable FLAG_PURE_NOT in external process, 2025-02-23)