servicemenuinstaller: Run installation scripts with cwd in their parent directories
Otherwise, if cwd is set to the unpacked dir root, some service menus
fail to install.
- Successfully Installed and uninstalled the "Color Folder" service menu from "Configure Dolphin..." -> service menus -> KNewStuff.
- ./test_run.rb still passes all its tests.
Reviewers: elvisangelaccio, sitter
Reviewed By: sitter
Differential Revision: https://phabricator.kde.org/D22466