Converting xpm to png fails silently (also on CI)

See https://gitlab.xfce.org/panel-plugins/xfce4-windowck-plugin/-/jobs/13370#L446.

echo -e "Going to convert xpm to png files"; \
./generator.py;
Going to convert xpm to png files
sh: 1: convert: not found
sh: 1: convert: not found
sh: 1: convert: not found
sh: 1: convert: not found
cp: cannot stat 'close_focused_normal.png': No such file or directory
sh: 1: convert: not found
sh: 1: convert: not found
sh: 1: convert: not found
sh: 1: convert: not found
cp: cannot stat 'maximize_focused_normal.png': No such file or directory
sh: 1: convert: not found
sh: 1: convert: not found
sh: 1: convert: not found
sh: 1: convert: not found
cp: cannot stat 'minimize_focused_normal.png': No such file or directory
sh: 1: convert: not found
sh: 1: convert: not found
sh: 1: convert: not found
sh: 1: convert: not found
cp: cannot stat 'menu_focused_normal.png': No such file or directory
sh: 1: convert: not found
sh: 1: convert: not found
sh: 1: convert: not found
sh: 1: convert: not found