Use `let_chains` to simplify some code in the oak containers launcher #807

Job	Run time
build_attest_all (buildconfigs/key_xor_test_app.sh)	41m 27s
build_attest_all (buildconfigs/oak_client_android_app.sh)	50m 0s
build_attest_all (buildconfigs/oak_containers_agent.sh)	36m 18s
build_attest_all (buildconfigs/oak_containers_kernel.sh)	35m 10s
build_attest_all (buildconfigs/oak_containers_orchestrator.sh)	49m 50s
build_attest_all (buildconfigs/oak_containers_stage1.sh)	49m 19s
build_attest_all (buildconfigs/oak_containers_syslogd.sh)	49m 59s
build_attest_all (buildconfigs/oak_containers_system_image.sh)	52m 3s
build_attest_all (buildconfigs/oak_containers_nvidia_system_image.sh)	52m 8s
build_attest_all (buildconfigs/oak_echo_enclave_app.sh)	41m 50s
build_attest_all (buildconfigs/oak_echo_raw_enclave_app.sh)	43m 37s
build_attest_all (buildconfigs/oak_functions_enclave_app.sh)	42m 44s
build_attest_all (buildconfigs/oak_functions_insecure_enclave_app.sh)	43m 3s
build_attest_all (buildconfigs/oak_orchestrator.sh)	42m 25s
build_attest_all (buildconfigs/oak_restricted_kernel_simple_io_init_rd_wrapper_bin.sh)	42m 20s
build_attest_all (buildconfigs/stage0_bin.sh)	43m 29s
build_attest_all (buildconfigs/stage0_bin_tdx.sh)	41m 52s
	12h 37m 34s

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