remove cargo install cargo-audit from CI (#4123)

ci/audit.sh

@@ -1,20 +0,0 @@
-#!/usr/bin/env bash
-#
-# Audits project dependencies for security vulnerabilities
-#
-set -e
-
-cd "$(dirname "$0")/.."
-source ci/_
-
-cargo_install_unless() {
-  declare crate=$1
-  shift
-
-  "$@" > /dev/null 2>&1 || \
-    _ cargo install "$crate"
-}
-
-cargo_install_unless cargo-audit cargo audit --version
-
-_ cargo audit

ci/test-checks.sh

@@ -12,7 +12,7 @@ export RUSTFLAGS="-D warnings"
 _ cargo +"$rust_stable" fmt --all -- --check
 _ cargo +"$rust_stable" clippy --all -- --version
 _ cargo +"$rust_stable" clippy --all -- --deny=warnings
-_ ci/audit.sh
+_ cargo +"$rust_stable" audit
 _ ci/nits.sh
 _ book/build.sh