Fix snap package build (1.4.x)

Make the snap build work again for 1.4.x.

See #5414

Some minor issues are remaining, which have probably been existing before. For example the inkman extension manager can install extensions but puts these into the standard extension directory where only the non-Snap version of Inkscape finds them. Also, it is not possible to select other variants of SVG (e.g. normal SVG vs Inkscape SVG) because the file-save dialog of Snap only supports one type per extension.