restic: add fish completions

  '' + lib.optionalString (stdenv.hostPlatform == stdenv.buildPlatform) ''

    $out/bin/restic generate \

      --bash-completion restic.bash \

      --fish-completion restic.fish \

      --zsh-completion restic.zsh \

      --man .

    installShellCompletion restic.{bash,zsh}

    installShellCompletion restic.{bash,fish,zsh}

    installManPage *.1

  '';