python3Packages.nextcord: fix issue with loading libopus

The substitution patch used to point nextcord to the installed location
of libopus in nix's store was incorrectly formatted:

* `@opus@` was used while default.nix attempted to substitute instances of
  `@libopus@`.
* `ctypes.util.find_library(<library_name>)` will return a path to a
  library. If we already have the path (provided by the substitution),
  then `find_library` is not necessary to call (nor will it produce the
  desired result).