CI: bump `cargo-audit` to v0.20 (#1090) #350

Job	Run time
build (1.73.0, thumbv7em-none-eabi)	25s
build (1.73.0, wasm32-unknown-unknown)	24s
build (stable, thumbv7em-none-eabi)	21s
cross (armv7-unknown-linux-gnueabihf, 1.73.0)	1m 43s
build (stable, wasm32-unknown-unknown)	14s
cross (armv7-unknown-linux-gnueabihf, stable)	1m 35s
doc	50s
test (i686-unknown-linux-gnu, 1.73.0, sudo apt update && sudo apt install gcc-multilib)	1m 30s
cross (aarch64-unknown-linux-gnu, 1.73.0)	1m 26s
test (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	1m 16s
cross (aarch64-unknown-linux-gnu, stable)	1m 7s
test (x86_64-unknown-linux-gnu, 1.73.0)	1m 13s
cross (powerpc-unknown-linux-gnu, 1.73.0)	1m 31s
test (x86_64-unknown-linux-gnu, stable)	57s
cross (powerpc-unknown-linux-gnu, stable)	1m 16s
	15m 48s

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