servicemenuinstaller: Run installation scripts with cwd in their parent directories

servicemenuinstaller: Run installation scripts with cwd in their parent directories

Summary:
Otherwise, if cwd is set to the unpacked dir root, some service menus
fail to install.

Test Plan:
 - 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

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D22466