CI: bump `cargo-audit` to v0.20 (#1695) #2615

Job	Run time
test (1.81.0)	19s
build (1.81.0, thumbv7em-none-eabi)	14s
test (stable)	11s
build (1.81.0, wasm32-unknown-unknown)	16s
build (stable, thumbv7em-none-eabi)	9s
build (stable, wasm32-unknown-unknown)	9s
	1m 18s

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