build: pull gimp submodules from github due to gitlab infra instability

[frenchwr]

Follow up on #300 to also pull gimp's submodules from github rather than gitlab as gitlab infra is under maintenance.

I tested the build locally and verified from the logs that the submodules are now being pulled from github:

2024-12-04 14:34:24.011 :: 2024-12-04 14:30:13.292 :: + sed -i 's|gitlab.gnome.org|github.com|' /root/parts/gimp/src/.gitmodules
2024-12-04 14:34:24.011 :: 2024-12-04 14:30:13.306 :: + git submodule update --init
2024-12-04 14:34:24.011 :: 2024-12-04 14:30:13.343 :: Submodule 'gimp-data' (https://github.com/GNOME/gimp-data.git) registered for path 'gimp-data'