Fix plugin_output for _generate_rust_gencode

[tempoz]

Used the python rules as an example to fix this. Bazel now recognizes that the output files have been produced, where before there were build failures regarding the outputs not being produced.

[JasonLunn]

Seeing failures on the Rust tests of the form:

Traceback (most recent call last):
	File "/workspace/rust/aspects.bzl", line 284, column 37, in _rust_cc_proto_aspect_impl
		return _rust_proto_aspect_common(target, ctx, is_upb = False)
	File "/workspace/rust/aspects.bzl", line 321, column 47, in _rust_proto_aspect_common
		(gencode, thunks) = _generate_rust_gencode(
	File "/workspace/rust/aspects.bzl", line 131, column 25, in _generate_rust_gencode
		proto_lang_toolchain.output_files = ctx.bin_dir.path + "/" + proto_root
Error: struct value does not support field assignment