Fix GraphicsMagick download link
The Polish mirror appears to be down. Use the SourceForge download link since http://www.graphicsmagick.org/download.html#download-sites claims "SourceForce has lots of bandwidth and many mirror sites."
https://gitlab.com/gitlab-org/build/CNG-mirror/-/jobs/1379248476 was failing, but now it's working again?
Edited by Stan Hu