Install Fish shell completions to /usr/share/fish/vendor_completions.d
    
    From https://fishshell.com/docs/current/completions.html:
    
    > By default, Fish searches the following for completions:
    >
    >  - ...
    >  - A directory for third-party software vendors to ship their own completions for their software,
         usually /usr/share/fish/vendor_completions.d;
    >  - The completions shipped with fish, usually installed in /usr/share/fish/completions;