[Services] Update commtest protobuf location

Summary:
Follow-up to D5046. These paths weren't referenced in any CI gate,
so they were overlooked.

Depends on D5101

Test Plan:

nix develop  # optional, if cargo + yarn is already installed
cd services/commtest
cargo clean && cargo build

Reviewers: varun, atul, abosh

Reviewed By: atul

Subscribers: ashoat, tomek, karol

Differential Revision: https://phab.comm.dev/D5102