Fix the krunner plugin settings access

Summary:
Within krunner our code looks inside krunnerrc and not zanshinrc picking
a wrong collection when saving created tasks. Override the file used by
KConfig for the time of the creation method then reset back to the
default config file. This way we make sure to use zanshinrc settings.

Reviewers: bensi, franckarrecot

Differential Revision: https://phabricator.kde.org/D1295