Alright, I've gotten it working.

After discussing on IRC with a few different people, I decided that 
making a new beets-specific variable was probably the best way forward. 
I've created $GUIX_BEETSPLUGINS for this purpose, which points to 
`/gnu/store/xxx-profile/lib/python3.9/site-packages`, and appended it to 
the $GUIX_PYTHONPATH which wraps 'beets'.

I've also wrapped the whole definition in a `let` to programmatically 
set the python version, so it is easier to update should it change. 
Ideally, this would be set from the input python's version, but I don't 
know how to do that (yet).

I've also taken the liberty of moving all of the 
unnecessarily-propagated inputs to 'beets-bandcamp' and made them normal 
inputs.

Patch is attached, let me know what You think.