[Tunnelbroker] Force finding protobuf config

Summary:
Headers from homebrew's protobuf were still being found.
Force find_library to find the related cmake CONFIG which should expose the
correct targets instead of just the library

example: https://buildkite.com/comm/tunnelbroker-unittests/builds/3

Test Plan: Tunnelbroker CI passes

Reviewers: max, atul

Reviewed By: atul

Subscribers: ashoat, tomek, abosh

Differential Revision: https://phab.comm.dev/D5139