Fix staking benchmark #13919

Job	Run time
preflight / preflight	43s
preflight / ci-versions	1m 2s
test-linux-stable-runtime-benchmarks	24m 47s
test-linux-stable-int	7m 32s
test-linux-stable (1/3, parity-large)	32m 17s
test-linux-stable (1/3, parity-oldlinux)	32m 33s
test-linux-stable (2/3, parity-large)	21m 26s
test-linux-stable-no-try-runtime (1/2)	32m 52s
test-linux-stable (2/3, parity-oldlinux)	18m 26s
test-linux-stable-no-try-runtime (2/2)	22m 37s
test-linux-stable (3/3, parity-large)	20m 32s
test-linux-stable (3/3, parity-oldlinux)	17m 14s
All tests passed	1s
	3h 52m 2s

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