Fix extended missing package add prompt

[IanButterworth]

#3038 broke the extended o prompt options

julia> using ProfileView
 │ Package ProfileView not found, but a package named ProfileView is available from a registry. 
 │ Install package?
 │   (Pkg) pkg> add ProfileView 
 └ Select environment:
   1: `~/Documents/GitHub/Pkg.jl/Project.toml` (@)
 > 2: `~/.julia/environments/v1.9/Project.toml` (@v#.#)
ERROR: UndefVarError: envs not defined
Stacktrace:
  [1] try_prompt_pkg_add(pkgs::Vector{Symbol})
    @ Pkg.REPLMode /Applications/Julia-1.9.app/Contents/Resources/julia/share/julia/stdlib/v1.9/Pkg/src/REPLMode/REPLMode.jl:747
...

Obviously that means the prompt needs better tests (#3083)

This PR just fixes the immediate issue