CI: bump `cargo-audit` to v0.20 (#622) #1057

Job	Run time
cross (1.71.0, aarch64-unknown-linux-gnu, default)	44s
cross (1.71.0, powerpc-unknown-linux-gnu, default)	1m 23s
cross (stable, aarch64-unknown-linux-gnu, default)	32s
cross (stable, powerpc-unknown-linux-gnu, default)	33s
minimal-versions / minimal-versions	51s
set-msrv / set-msrv	3s
test (1.71.0)	1m 3s
build (1.71.0, thumbv7em-none-eabi)	21s
test (stable)	45s
build (1.71.0, wasm32-unknown-unknown)	24s
build (stable, thumbv7em-none-eabi)	14s
build (stable, wasm32-unknown-unknown)	37s
	7m 30s

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