Overhaul cargo/rustc version management

ci/test-checks.sh

@@ -4,14 +4,14 @@ set -e
 cd "$(dirname "$0")/.."
 
 source ci/_
-ci/version-check.sh stable
+source ci/rust-version.sh stable
 
 export RUST_BACKTRACE=1
 export RUSTFLAGS="-D warnings"
 
-_ cargo fmt --all -- --check
-_ cargo clippy --all -- --version
-_ cargo clippy --all -- --deny=warnings
+_ cargo +"$rust_stable" fmt --all -- --check
+_ cargo +"$rust_stable" clippy --all -- --version
+_ cargo +"$rust_stable" clippy --all -- --deny=warnings
 _ ci/audit.sh
 _ ci/nits.sh
 _ book/build.sh