CI: bump `cargo-audit` to v0.20 (#1695) #1809

Job	Run time
test (1.81.0)	20s
build (1.81.0, thumbv7em-none-eabi)	15s
test (stable)	12s
build (1.81.0, wasm32-unknown-unknown)	15s
build (stable, thumbv7em-none-eabi)	8s
build (stable, wasm32-unknown-unknown)	15s
minimal-versions / minimal-versions	22s
	1m 47s

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