CI: bump `cargo-audit` to v0.20 (#1090) #452

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	30s
build (1.73.0, wasm32-unknown-unknown)	26s
doc	53s
build (stable, thumbv7em-none-eabi)	21s
build (stable, wasm32-unknown-unknown)	22s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 8s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	54s
test (x86_64-unknown-linux-gnu, 1.73.0)	49s
test (x86_64-unknown-linux-gnu, stable)	38s
	6m 1s

Provide feedback