Fix vsync race condition on providerscope unmount #4706

Job	Run time
build (stable, examples/counter)	1m 10s
build (stable, examples/marvel)	58s
build (stable, examples/pub)	1m 2s
build (stable, examples/random_number)	1m 6s
build (stable, examples/stackoverflow)	54s
build (stable, examples/todos)	1m 13s
build (stable, packages/riverpod)	2m 52s
build (stable, packages/riverpod/example)	56s
build (stable, packages/flutter_riverpod)	1m 24s
build (stable, packages/flutter_riverpod/example)	1m 7s
build (stable, packages/hooks_riverpod)	1m 27s
build (stable, packages/hooks_riverpod/example)	1m 11s
build (stable, packages/riverpod_annotation)	57s
build (stable, packages/riverpod_generator)	2m 6s
build (stable, packages/riverpod_generator/integration/build_yaml)	1m 42s
build (stable, website)	51s
build (master, examples/counter)	1m 44s
build (master, examples/marvel)	1m 57s
build (master, examples/pub)	1m 33s
build (master, examples/random_number)	1m 21s
build (master, examples/stackoverflow)	1m 43s
build (master, examples/todos)	1m 28s
build (master, packages/riverpod)	3m 13s
build (master, packages/riverpod/example)	1m 12s
build (master, packages/flutter_riverpod)	1m 51s
build (master, packages/flutter_riverpod/example)	1m 28s
build (master, packages/hooks_riverpod)	1m 33s
build (master, packages/hooks_riverpod/example)	1m 52s
build (master, packages/riverpod_annotation)	1m 6s
build (master, packages/riverpod_generator)	1m 57s
build (master, packages/riverpod_generator/integration/build_yaml)	2m 41s
build (master, website)	1m 27s
	49m 2s

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