CI: bump `cargo-audit` to v0.20 (#1569) #1091

Job	Run time
build (1.81.0, thumbv7em-none-eabi)	25s
minimal-versions	0s
build (1.81.0, wasm32-unknown-unknown)	29s
test (1.81.0)	26s
build (stable, thumbv7em-none-eabi)	26s
test (stable)	17s
build (stable, wasm32-unknown-unknown)	17s
	2m 20s

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