Pluto package not included in Pluto Package Manager manifest

This README page has a link to Binder to view my Pluto notebook.

That notebook uses some unregistered packages, so I used Pluto.activate_notebook_environment from a Julia REPL to set up its Pluto Package Manager Project file and Manifest.

When I open that notebook link in Binder, I get the error

ArgumentError: Package Pluto not found in current path:

- Run `import Pkg; Pkg.add("Pluto")` to install the Pluto package.

require(::Module, ::Symbol)@loading.jl:967
top-level scope@Local: 2

I tried doing an explicit Pkg.add("Pluto") from the REPL to the notebook environment but Pluto dod not get added to the manifest.

Were I to put Pkg.add("Pluto") directly in a notebook cell then the user would need to endure the time overhead of Julia package resolution.

It seems like the Pluto Package Manager is specifically exclusding Pluto from the notebook manifest.

How do I make this work?