fzf: change debian completion file path (#8402)

* Newer Debian packages install completions file in
/usr/share/doc/fzf/examples/completion.zsh

* Default to buster/stretch path if completion file not found

See file list in order from older to newer fzf package versions:
- https://packages.debian.org/stretch-backports/amd64/fzf/filelist
- https://packages.debian.org/buster/amd64/fzf/filelist
- https://packages.debian.org/bullseye/amd64/fzf/filelist
- https://packages.debian.org/sid/amd64/fzf/filelist



Co-authored-by: Marc Cornellà <marc.cornella@live.com>