CI: bump `cargo-audit` to v0.20 (#1090) #736

Job	Run time
doc	40s
test (1.73.0)	26s
test (stable)	19s
build (1.73.0, thumbv7em-none-eabi)	24s
build (1.73.0, wasm32-unknown-unknown)	23s
build (stable, thumbv7em-none-eabi)	14s
build (stable, wasm32-unknown-unknown)	13s
	2m 39s

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