Fix first run of appimage creation: get install_colorschemes.py via $SRC

Summary:
On the first run of kdevelop-recipe-centos6.sh it is run from a copied
version, not the git checkout of kdevelop.

Reviewers: KDevelop, zhigalin

Reviewed By: KDevelop, zhigalin

Subscribers: kfunk, brauch, zhigalin, kdevelop-devel

Tags: KDevelop

Differential Revision: https://phabricator.kde.org/D17581