so there is a chance i'm being silly, but i checked out 394b1e7, ran

rm -rf ~/.cache/pants/rust-toolchain; ./pants -V

in the pants repo: it recompiled all these cargo packages, then i got the same error as before ("failed to update submodule grpc-sys/grpc"), while trying to download submodules of

illicitonion/grpc-rs?rev=2acabd...

.