CI: bump `cargo-audit` to v0.20 (#1695) #2138

Job	Run time
build (1.65.0, thumbv7em-none-eabi)	1m 9s
build (1.65.0, wasm32-unknown-unknown)	1m 15s
build (stable, thumbv7em-none-eabi)	18s
minimal-versions	19s
build (stable, wasm32-unknown-unknown)	11s
test (1.65.0)	1m 13s
test (stable)	11s
	4m 36s

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