CI: bump `cargo-audit` to v0.20 (#622) #659

Job	Run time
minimal-versions / minimal-versions	34s
set-msrv / set-msrv	6s
test (1.71.0)	39s
test (stable)	37s
build (1.71.0, thumbv7em-none-eabi)	25s
build (1.71.0, wasm32-unknown-unknown)	21s
build (stable, thumbv7em-none-eabi)	14s
build (stable, wasm32-unknown-unknown)	14s
	3m 10s

Provide feedback