Makefile: Include pyln-grpc proto in PYTHONPATH

When running `make check-python` we'll expclicitly include
`pyln-grpc-proto` in the `PYTHONPATH`.

This ensures we always run the tests using the version installed in
`./contrib/pyln-grpc-proto`

Makefile

@@ -72,7 +72,7 @@ endif
 
 # (method=thread to support xdist)
 PYTEST_OPTS := -v -p no:logging $(PYTEST_OPTS)
-MY_CHECK_PYTHONPATH=$${PYTHONPATH}$${PYTHONPATH:+:}$(shell pwd)/contrib/pyln-client:$(shell pwd)/contrib/pyln-testing:$(shell pwd)/contrib/pyln-proto/:$(shell pwd)/contrib/pyln-spec/bolt1:$(shell pwd)/contrib/pyln-spec/bolt2:$(shell pwd)/contrib/pyln-spec/bolt4:$(shell pwd)/contrib/pyln-spec/bolt7
+MY_CHECK_PYTHONPATH=$${PYTHONPATH}$${PYTHONPATH:+:}$(shell pwd)/contrib/pyln-client:$(shell pwd)/contrib/pyln-testing:$(shell pwd)/contrib/pyln-proto/:$(shell pwd)/contrib/pyln-spec/bolt1:$(shell pwd)/contrib/pyln-spec/bolt2:$(shell pwd)/contrib/pyln-spec/bolt4:$(shell pwd)/contrib/pyln-spec/bolt7:$(shell pwd)/contrib/pyln-grpc-proto
 # Collect generated python files to be excluded from lint checks
 PYTHON_GENERATED= \
 	contrib/pyln-grpc-proto/pyln/grpc/primitives_pb2.py \