nullglob bad-example leaks temp dir under Bash 3.2 (EXIT trap in subshell)

[posidoni]

What's wrong

examples/bash/04-nullglob-empty-match.bad.sh defines mktemp -d and
trap 'rm -rf "$tmp"' EXIT inside the main() ( ... ) subshell:

main() (
  tmp=$(mktemp -d)
  trap 'rm -rf "$tmp"' EXIT
  cd "$tmp"

  for file in *.txt; do
    printf 'found: %s\n' "$file"
  done
)

Under Bash 3.2 — which macOS ships by default and which this repo's
shell-macos CI job explicitly targets — an EXIT trap defined inside a
subshell (...) does not fire when the subshell exits. The temp directory
is created but never cleaned up, leaking on every run (including every CI run
of tests/examples.bats on the macOS job).

Flagged by gemini-code-assist on PR #12, and already fixed on the paired
.good.sh in the same PR (see the merged commit) by hoisting tmp/trap to
global scope:

tmp=$(mktemp -d)
trap 'rm -rf "$tmp"' EXIT

main() (
  cd "$tmp"
  ...
)

What to do

Apply the identical hoist to examples/bash/04-nullglob-empty-match.bad.sh
so both files in the pair use the same (correct) temp-dir/trap scope. No
behavioral change to the demonstrated pitfall (the unmatched-glob-stays-literal
behavior is unaffected) — this is purely a cleanup/portability fix.

Verify

task ci

must stay green.