CI: Remove unnecessary toolchain install step #5209

Job	Run time
check	47s
check-docs	17s
cargo-hack	3m 5s
cargo-public-api-crates (axum)	31s
dependencies-are-sorted	13s
cargo-deny check (advisories)	24s
cargo-public-api-crates (axum-core)	16s
cargo-deny check (bans licenses sources)	23s
cargo-public-api-crates (axum-extra)	29s
Spell Check with Typos	0s
cargo-public-api-crates (axum-macros)	14s
test-docs	1m 26s
test-msrv	2m 9s
test-nightly	1m 39s
armv5te-unknown-linux-musleabi	26s
test-versions (stable)	1m 38s
test-versions (beta)	1m 42s
wasm32-unknown-unknown	11s
	15m 50s

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