CI: bump `cargo-audit` to v0.20 (#622) #256

Job	Run time
minimal-versions / minimal-versions	22s
set-msrv / set-msrv	3s
test (1.71.0)	17s
build (1.71.0, thumbv7em-none-eabi)	26s
test (stable)	26s
build (1.71.0, wasm32-unknown-unknown)	18s
build (stable, thumbv7em-none-eabi)	26s
build (stable, wasm32-unknown-unknown)	14s
	2m 32s

Provide feedback