[CMake] Use correct protobuf capitalization

Summary:
Fix failure from D4998#14767.

Correctly capitalizing the package name prevents cmake from
visiting all potential cmake configs and polluting the
environment with potentially conflicting packages outside
of the nix environment.

Test Plan:

nix develop
cd services/blob
rm -rf build && mkdir build && cd build && cmake .. && make -j

Reviewers: atul, abosh, varun

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5106