do not show silly mounts in sandboxed envs

Harald Sitter requested to merge work/noflatmounts into release/22.04

inside sandboxes we get all sorts of silly mounts that have no real value. since the list of potential mounts is fairly long I've opted to skip over all readonly mounts as well as all of /var and a special app-specific var variant. inside flatpak that covers everything of no value pretty much

BUG: 450835

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