malus: use spawn_blocking (#2804)

Looks like using `spawn` instead of `spawn_blocking` in malus is leading
to a deadlock somehow. I'm not sure why exactly, but switching to
`spawn_blocking` fixes
https://github.com/paritytech/disabling-e2e-tests/issues/1, at least for
`suggest-garbage-candidate` (didn't check other variants).

Maybe my assumption here was wrong:
https://github.com/paritytech/polkadot-sdk/pull/2184#discussion_r1403587674.

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Co-authored-by: Tsvetomir Dimitrov <[email protected]>