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>

plugins/fzf/fzf.plugin.zsh

@@ -67,7 +67,10 @@ function setup_using_debian_package() {
     # NOTE: There is no need to configure PATH for debian package, all binaries
     # are installed to /usr/bin by default
 
-    local completions="/usr/share/zsh/vendor-completions/_fzf"
+    # Determine completion file path: first bullseye/sid, then buster/stretch
+    local completions="/usr/share/doc/fzf/examples/completion.zsh"
+    [[ -f "$completions" ]] || completions="/usr/share/zsh/vendor-completions/_fzf"
+
     local key_bindings="/usr/share/doc/fzf/examples/key-bindings.zsh"
 
     # Auto-completion