CI: bump `cargo-audit` to v0.20 (#1569) #1536

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	24s
build (1.81.0, wasm32-unknown-unknown)	23s
build (stable, thumbv7em-none-eabi)	19s
test (1.81.0)	1m 24s
build (stable, wasm32-unknown-unknown)	17s
test (stable)	1m 15s
minimal-versions	0s
	4m 2s

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