pw_protobuf: Replace deprecated API usage

Bug: 253313672
Change-Id: Iaabd9589116385c64ebf8ba0eda527449eb7b0da
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/123235
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Ted Pudlik <tpudlik@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>

pw_protobuf_compiler/pw_proto_library.bzl

@@ -326,7 +326,7 @@ def _proto_compiler_aspect_impl(target, ctx):
     args.add_joined(
         "--descriptor_set_in",
         target[ProtoInfo].transitive_descriptor_sets,
-        join_with = ctx.host_configuration.host_path_separator,
+        join_with = ctx.configuration.host_path_separator,
         map_each = _get_path,
     )