Add webp-pixbuf-loader

PBSrequested to merge
pbs3141/inkscape-ci-docker:webp into master

Adds webp support to the AppImage, as part of inkscape#5089

Also switch the few usages of apt install to be apt-get install like the rest of them.

Confirmed not to break Inkscape CI at https://gitlab.com/pbs3141/inkscape/-/pipelines/2632876113

Edited by PBS

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