Fixes#705
GUI launches of the AppImage on Linux do not inherit the shell PATH, so
`find_on_path` and `find_in_user_shell` cannot reach Homebrew-on-Linux's
Node install. Add the canonical Linuxbrew paths to `fallback_node_paths`
and to `node_binary_candidates_for_home`; `.is_file()` filtering keeps
machines without Linuxbrew unaffected.