Julia 1.11 compat: Use Base.BufferStream for capturing Pkg IO

[fonsp]

implement #2910

[github-actions]

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="Pkg-IO-capture-BufferStream")
julia> using Pluto

[fonsp]

This PR seems to fix the issue! Except it stalls forever when doing Pkg operations, unless I add lots of logs to debug this, and then it magically works. So I need to figure out which log fixed it and why? (Maybe I need a yield or something?)

[fonsp]

pankgeorg suggested flush(some_io)

[fonsp]

The problem was that I was doing readavailable on a stream that was already fully read, so it was waiting forever for the next byte. The logs "fixed" it because the logs were also printed in this stream, adding bytes.

[fonsp]

This fixed it! There is one more Pkg issue that I will maybe do in a separate PR

[fonsp]

last failure is julia-actions/setup-julia#240