Avoid using lambda in `skylet.constant` #15973

Job	Run time
python-test (3.8, tests/unit_tests)	6m 12s
python-test (3.8, tests/test_api.py)	3m 51s
python-test (3.8, tests/test_cli.py)	4m 27s
python-test (3.8, tests/test_config.py)	3m 57s
python-test (3.8, tests/test_global_user_state.py)	3m 45s
python-test (3.8, tests/test_jobs.py)	4m 31s
python-test (3.8, tests/test_list_accelerators.py)	4m 8s
python-test (3.8, tests/test_optimizer_dryruns.py)	5m 32s
python-test (3.8, tests/test_optimizer_random_dag.py)	4m 25s
python-test (3.8, tests/test_storage.py)	3m 51s
python-test (3.8, tests/test_wheels.py)	3m 46s
python-test (3.8, tests/test_jobs_and_serve.py)	3m 43s
python-test (3.8, tests/test_yaml_parser.py)	4m 6s
	56m 14s

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