Use scratch space for notebook package environment directory

[fonsp]

The directory where Project.toml + Manifest.toml is (secretly) stored for a running notebook is current a mktempdir() folder. This PR uses Scratch.jl for that, to fix #2641

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="scratch-for-notebook-env")
julia> using Pluto

[fonsp]

This can still lead to issues when running a notebook for more than 7 days...