Harden CI tox (3.13) against transient ghdl/setup-ghdl download failures

[Copilot]

The tox (3.13) GitHub Actions job was failing before tox execution because ghdl/setup-ghdl@v1 intermittently returned HTTP 503 while fetching release inventory assets. This change makes the workflow resilient by preserving the existing setup path and adding a conditional Ubuntu package fallback.

• Failure handling in GHDL setup

  • Added id: setup-ghdl and continue-on-error: true to the ghdl/setup-ghdl@v1 step.
  • Keeps the current preferred installer as first choice.

• Conditional fallback path

  • Added a fallback step gated on steps.setup-ghdl.outcome == 'failure'.
  • Installs GHDL from apt (ghdl-llvm, fallback to ghdl) so downstream tox jobs can proceed when GitHub release endpoints are temporarily unavailable.

- id: setup-ghdl
  uses: ghdl/setup-ghdl@v1
  continue-on-error: true
  with:
    backend: llvm

- name: Fallback install GHDL from apt
  if: steps.setup-ghdl.outcome == 'failure'
  run: |
    sudo apt-get update
    sudo apt-get install -y ghdl-llvm || sudo apt-get install -y ghdl
    ghdl --version