Rename `assigner_on_demand` to `on_demand` (#4706) #2

Job	Run time
set-image	9s
check-workspace	17s
check-dependency-rules	8s
check-markdown	15s
check-rust-feature-propagation	2m 16s
test-rust-features	2m 23s
check-toml-format	1m 16s
check-umbrella	2m 26s
fmt	1m 5s
	10m 15s

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