feat: reinstate rustfmt script, detach from CI

scripts/rust_fmt.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+
+set -euo pipefail
+
+if [[ -n "${CI:-}" ]]; then
+  echo "This script should not be run in a CI environment, as it installs toolchains out of cache."
+  exit 1
+fi
+
+SCRIPT_DIR="$(dirname "${BASH_SOURCE[0]}")"
+TOOLCHAIN=$(grep "EXTRA_RUST_TOOLCHAINS:" "${SCRIPT_DIR}"/../.github/workflows/main.yml  | awk '{print $2}')
+
+function install_rustfmt() {
+    rustup toolchain install "${TOOLCHAIN}"
+    rustup component add --toolchain "${TOOLCHAIN}" rustfmt
+}
+
+rustup toolchain list | grep -q "${TOOLCHAIN}" || install_rustfmt
+
+cargo +"${TOOLCHAIN}" fmt "$@"