qt: Start dirmngr with gpgconf to avoid multiple instances
* lang/qt/src/qgpgmewkdlookupjob.cpp (startDirmngr): Use "gpgconf --launch dirmngr" to start dirmngr.
Using gpgconf to start dirmngr prevents multiple instances to be started
by the concurrently running keyserver lookup and WKD lookup.
- GnuPG-bug-id: T6833