Add soffice for the libreoffice binary name

LibreOffice is not available on every platform under the libreoffice binary, but the soffice binary seems to be present everywhere. This came up in the nixpkgs context, but since the soffice binary also seems to present on other platforms this patch seems worth upstreaming.

Note on the failing pipeline: I was not able to get the Gitlab CI of my fork Working (seems to be failing around fetching the docker container). But since the contained test succeed in the nixpkgs-packaging with these patches, I'm reasonably confident that this should not introduce any regressions.