CI: bump `cargo-audit` to v0.20 (#1695) #2081

Job	Run time
minimal-versions	33s
build (1.60.0, thumbv7em-none-eabi)	1m 9s
build (1.60.0, wasm32-unknown-unknown)	1m 8s
test (1.60.0)	1m 18s
build (stable, thumbv7em-none-eabi)	10s
test (stable)	32s
build (stable, wasm32-unknown-unknown)	11s
	5m 1s

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