ok, i added

--plugin=protoc-gen-rust="${HOME}/.cache/pants/rust/cargo/bin/protoc-gen-rust"

to

src/rust/engine/process_execution/bazel_protos/generate-grpc.sh

and

cargo build

worked again (i removed

~/.cargo/bin

from my path and removed the manually cargo-installed

protoc-gen-rust

before checking if the build worked again)