Don't concatenate paths in --proto_path (#109)

`protoc`'s `--proto_path` argument is expected to be provided multiple
times if multiple directories are to be considered. Concatenating with
`:` is invalid on some systems (Windows):

```sh
> protoc --help
  -IPATH, --proto_path=PATH   Specify the directory in which to search for
                              imports.  May be specified multiple times;
                              directories will be searched in order.  If not
                              given, the current working directory is used.
                              If not found in any of the these directories,
                              the --descriptor_set_in descriptors will be
                              checked for required proto file.
```

src/substrait/proto/CMakeLists.txt

@@ -66,8 +66,8 @@ foreach(PROTO_FILE IN LISTS PROTOBUF_FILELIST)
  add_custom_command(
  OUTPUT ${PROTO_SRC} ${PROTO_HDR}
  COMMAND
- protobuf::protoc
- "--proto_path=${GENERATED_PROTO_TOPLEVEL_DIR}:${protobuf_SOURCE_DIR}/src"
+ protobuf::protoc "--proto_path=${GENERATED_PROTO_TOPLEVEL_DIR}"
+ "--proto_path=${protobuf_SOURCE_DIR}/src"
  "--cpp_out=${PROTO_OUTPUT_MIDLEVEL_DIR}" ${GENERATED_PROTO_FILE}
  DEPENDS ${GENERATED_PROTOBUF_LIST} protobuf::protoc
  COMMENT "Generated C++ protobuf module for ${PROTO_FILE}"