CI flake in pre-build checks

[daywalker90]

there is a race condition between sed rewriting node_pb2.py and node_pb2_grpc.py and the git diff's from check-source causing the prebuild CI step to fail with:

--- a/contrib/pyln-grpc-proto/pyln/grpc/node_pb2_grpc.py
+++ b/contrib/pyln-grpc-proto/pyln/grpc/node_pb2_grpc.py
@@ -3,7 +3,7 @@
 import grpc
 import warnings

-from pyln.grpc import node_pb2 as node__pb2
+import node_pb2 as node__pb2