qt: Pass home directory with forward slashes when launching dirmngr

* lang/qt/src/qgpgmewkdlookupjob.cpp (startDirmngr): Replace backslashes
with forward slashes in homedir returned by GpgME::dirInfo.

This works around a bug which causes backslashes to be escaped (doubled)
twice on Windows, once by gpgme_op_spawn and once by the spawn helper
gpgme-w32spawn. The next versions of GnuPG 2.2 and 2.4 also have a
workaround for this.

• GnuPG-bug-id: T6833