Add Kokoro git repo to safe dirs #293

Job	Run time
build_attest_all (buildconfigs/key_xor_test_app.sh)	4m 20s
build_attest_all (buildconfigs/oak_client_android_app.sh)	16m 25s
build_attest_all (buildconfigs/oak_containers_agent.sh)	5m 39s
build_attest_all (buildconfigs/oak_containers_kernel.sh)	4m 12s
build_attest_all (buildconfigs/oak_containers_orchestrator.sh)	8m 59s
build_attest_all (buildconfigs/oak_containers_stage1.sh)	5m 56s
build_attest_all (buildconfigs/oak_containers_syslogd.sh)	6m 50s
build_attest_all (buildconfigs/oak_containers_system_image.sh)	14m 46s
build_attest_all (buildconfigs/oak_containers_nvidia_system_image.sh)	16m 52s
build_attest_all (buildconfigs/oak_echo_enclave_app.sh)	3m 53s
build_attest_all (buildconfigs/oak_echo_raw_enclave_app.sh)	4m 28s
build_attest_all (buildconfigs/oak_functions_enclave_app.sh)	4m 12s
build_attest_all (buildconfigs/oak_functions_insecure_enclave_app.sh)	4m 40s
build_attest_all (buildconfigs/oak_orchestrator.sh)	3m 51s
build_attest_all (buildconfigs/oak_restricted_kernel_simple_io_init_rd_wrapper_bin.sh)	6m 26s
build_attest_all (buildconfigs/stage0_bin.sh)	5m 58s
	1h 57m 27s

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