Update Rust before setting up the cache

[mgeisler]

The cache is keyed on the Rust version used, so we should update Rust before setting up the cache.

This is a small update to #2449.

[mgeisler]

Looking a bit more at rustup, I wonder if we should start using a toolchain file: https://rust-lang.github.io/rustup/overrides.html#the-toolchain-file.

I've never had to do this before, but it seems like it could simplify our setup in a few places? It could perhaps make

comprehensive-rust/.github/workflows/build.yml

Lines 70 to 81 in f888219

	matrix:
	include:
	- directory: src/bare-metal/alloc-example
	target: aarch64-unknown-none
	- directory: src/bare-metal/aps/examples
	target: aarch64-unknown-none
	- directory: src/bare-metal/microcontrollers/examples
	target: thumbv7em-none-eabihf
	- directory: src/exercises/bare-metal/compass
	target: thumbv7em-none-eabihf
	- directory: src/exercises/bare-metal/rtc
	target: aarch64-unknown-none

unnecessary (by moving this into separate toolchain files).