CI: bump `cargo-audit` to v0.20 (#622) #1026

Job	Run time
minimal-versions / minimal-versions	23s
set-msrv / set-msrv	3s
test (1.71.0)	23s
build (1.71.0, thumbv7em-none-eabi)	18s
test (stable)	24s
build (1.71.0, wasm32-unknown-unknown)	18s
build (stable, thumbv7em-none-eabi)	12s
build (stable, wasm32-unknown-unknown)	18s
	2m 19s

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