Move finding protobuf include to oak_proto_build_utils. #107

Job	Run time
build_attest_all (buildconfigs/key_xor_test_app.sh)	2m 14s
build_attest_all (buildconfigs/oak_client_android_app.sh)	43m 21s
build_attest_all (buildconfigs/oak_containers_kernel.sh)	24m 17s
build_attest_all (buildconfigs/oak_containers_orchestrator.sh)	5m 29s
build_attest_all (buildconfigs/oak_containers_stage1.sh)	3m 40s
build_attest_all (buildconfigs/oak_containers_syslogd.sh)	4m 39s
build_attest_all (buildconfigs/oak_containers_agent.sh)	3m 12s
build_attest_all (buildconfigs/oak_containers_system_image.sh)	11m 26s
build_attest_all (buildconfigs/oak_echo_enclave_app.sh)	2m 10s
build_attest_all (buildconfigs/oak_echo_raw_enclave_app.sh)	2m 7s
build_attest_all (buildconfigs/oak_functions_enclave_app.sh)	2m 38s
build_attest_all (buildconfigs/oak_functions_insecure_enclave_app.sh)	2m 29s
build_attest_all (buildconfigs/oak_orchestrator.sh)	2m 24s
build_attest_all (buildconfigs/oak_restricted_kernel_simple_io_init_rd_wrapper_bin.sh)	2m 42s
build_attest_all (buildconfigs/stage0_bin.sh)	2m 53s
	1h 55m 41s

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