[Services] Update protobuf location for blob_client

Summary:
Blob CI uses the docker workflow which was fixed, this in-repo change
was overlooked in D4998.

Depends on D5102

Test Plan:

nix develop
cd service/backup/
rm -rf build && mkdir build && cd build && cmake .. && make -j

Reviewers: atul, varun, abosh, karol

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5103