CI: unblock test (clang++-10, clang-10) by handling PEP 668 pip installs

[Copilot]

The test (clang++-10, clang-10) GitHub Actions job fails in the unit-test step because containerized Python is now PEP 668-managed, and plain pip install exits with externally-managed-environment. This PR updates the workflow to install test Python deps in a way compatible with current Ubuntu/Python packaging behavior.

• Root cause addressed

  • Unit-test dependency installation in .github/workflows/test.yml used system-level pip install without PEP 668 override.

• Workflow change

  • Updated the unit-test install command to pass --break-system-packages to pip in the Docker test container.
  • Scope is limited to the CI unit-test dependency install line; no matrix, build, or test command changes.

• Patch excerpt

  - python3 -m pip install ptf scapy grpcio googleapis-common-protos protobuf==3.20
  + python3 -m pip install --break-system-packages ptf scapy grpcio googleapis-common-protos protobuf==3.20