CI: bump `cargo-audit` to v0.20 (#1569) #1439

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 13s
build (1.65.0, wasm32-unknown-unknown)	1m 7s
test (1.65.0)	1m 28s
build (stable, thumbv7em-none-eabi)	13s
build (stable, wasm32-unknown-unknown)	12s
test (stable)	30s
minimal-versions / minimal-versions	26s
	5m 9s

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