beets: Use pythonPackages.buildPythonApplication

The top-level attribute has been removed in commit
771ed59b.

This has been partially resolved in commit
18bdd447

.

The latter change however only addressed the main derivations but missed
out on the plugins. This is now done by just passing pythonPackages down
the chain.

Tested by only evaluating the expression, not building.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Cc: @domenkozar, @pjones, @profpatsch