Change the effective proto root for C#-specific test protos.

PiperOrigin-RevId: 505928634

csharp/generate_protos.sh

diff --git a/csharp/generate_protos.sh b/csharp/generate_protos.sh
index 8be4428..cd682c2 100755
--- a/csharp/generate_protos.sh
+++ b/csharp/generate_protos.sh
@@ -50,7 +50,7 @@
 # Note that this deliberately does *not* include old_extensions1.proto
 # and old_extensions2.proto, which are generated with an older version
 # of protoc.
-$PROTOC -Isrc -Icsharp/protos \
+$PROTOC -Isrc -I. \
     --experimental_allow_proto3_optional \
     --csharp_out=csharp/src/Google.Protobuf.Test.TestProtos \
     --csharp_opt=file_extension=.pb.cs \