CI: add bounded retry loop for KiCad package install in schematic export workflow

[electricalhog]

The schematic export workflow was fragile on transient apt/network failures while installing KiCad. This change makes the install step self-retrying so the job can recover automatically without manual reruns.

• What changed

  • Replaced the single-shot KiCad install with a bounded retry loop in export-schematic.yml.
  • Each attempt runs apt-get update + apt-get install kicad kicad-symbols atomically.
  • Workflow now fails explicitly only after the final retry attempt.

• Retry behavior

  • 3 total attempts.
  • Progressive backoff between attempts (10s, 20s).
  • Clear terminal message on terminal failure.

for attempt in 1 2 3; do
  echo "KiCad install attempt ${attempt}/3"
  if sudo apt-get update -qq && sudo apt-get install -y --no-install-recommends kicad kicad-symbols; then
    exit 0
  fi
  if [ "${attempt}" -eq 3 ]; then
    echo "KiCad installation failed after 3 attempts."
    exit 1
  fi
  sleep "$((attempt * 10))"
done