static_tests/rust: Address shellcheck lints

The pure POSIX way to do `for x in $(find ...)` right involves a
tempfile, thus limiting to bash.

dist/tools/cargo-checks/check.sh

@@ -1,14 +1,10 @@
-#!/bin/sh
+#!/bin/bash
 
 set -eu
 
 FAILURES=""
 
-# This is a Make based environment, therefore we don't have funny names to take
-# care of, and anyone who can cause commands to be run by injecting files can
-# just as well modify the scripts being run. (Otherwise we'd need to go through
-# a less readable print0 / read).
-for CARGOTOML in `find -name Cargo.toml`
+find . -name Cargo.toml -print0 | while IFS= read -r -d '' CARGOTOML
 do
     if cargo fmt --quiet --check --manifest-path "${CARGOTOML}"; then
         continue
@@ -23,7 +19,7 @@ if [ x"" != x"${FAILURES}" ]; then
     echo "You can format the code locally using:"
     echo
     echo "find -name Cargo.toml -exec cargo fmt --manifest-path '{}' ';'"
-    if [ ! -z "${GITHUB_RUN_ID:-}" ]; then
+    if [ -n "${GITHUB_RUN_ID:-}" ]; then
         echo
         echo "The author of this test regrets not knowing how to provide an easy way to just click a button here that provides the right fixup commits."
     fi