[GCP] Fix `--disk-size` for Custom Machine Images #10717

Job	Run time
python-test (3.8, tests/unit_tests)	6m 35s
python-test (3.8, tests/test_cli.py)	4m 1s
python-test (3.8, tests/test_config.py)	3m 37s
python-test (3.8, tests/test_global_user_state.py)	3m 30s
python-test (3.8, tests/test_jobs.py)	4m 16s
python-test (3.8, tests/test_list_accelerators.py)	3m 35s
python-test (3.8, tests/test_optimizer_dryruns.py)	6m 11s
python-test (3.8, tests/test_optimizer_random_dag.py)	5m 39s
python-test (3.8, tests/test_storage.py)	3m 46s
python-test (3.8, tests/test_wheels.py)	3m 4s
python-test (3.8, tests/test_spot.py)	3m 13s
python-test (3.8, tests/test_yaml_parser.py)	3m 17s
	50m 44s

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