CI: bump `cargo-audit` to v0.20 (#182) #125

Job	Run time
test (1.61)	1m 29s
build (1.61, thumbv7em-none-eabi)	1m 9s
test (stable)	22s
build (1.61, wasm32-unknown-unknown)	1m 7s
build (stable, thumbv7em-none-eabi)	9s
build (stable, wasm32-unknown-unknown)	9s
	4m 25s

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