Fix appimagetool usage in docker

[cajomar]

Invocation from docker was failing because the generate_type2_appimage function was attempting to operate on a local path appimagetool instead of expanding the $appimagetool variable, giving this error:

mv: cannot stat 'appimagetool': No such file or directory

Adding this line to my script fixed my job:

sed -i -e 's/mv appimagetool/mv $appimagetool/' /opt/pkg2appimage.AppDir/usr/share/pkg2appimage/functions.sh

Full CI script: https://gitlab.com/cajomar/snowballz/-/blob/59fc8c35168a007a064e63885425a47278bbb989/.gitlab-ci.yml

[probonopd]

Thanks @cajomar, looks reasonable.