[CI] Prevent all protobuf tests from being build

Summary:
Although BUILD_TESTING is referenced in some of the
cmake logic for protobuf, the main switch is protobuf_BUILD_TESTS.

Part of https://linear.app/comm/issue/ENG-2721/

Test Plan: CI gate should not build the testing executable

Reviewers: atul, varun, max, bartek, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D6288