Set the icon name to ${LINUX_APPLICATION_ID} on Linux

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### Feature description
In Flatpak, the exported application icon name is the LINUX_APPLICATION_ID so any other desktop component will only know this icon name (so com.nextcloud.desktopclient.nextcloud not Nextcloud for instance).
I believe that it would be the same issue with any other containerized format.
It would then probably make sense to just use this as the default icon name (at least on Linux) so that we don't have to patch the Nextcloud Desktop application for our needs.