Use POSIX-compatible conditionals in install test helper invoked via sh

[Copilot]

The linked review comment flagged bash-only [[ ... ]] usage in tests/docker/install/_helper_run_install_tests.sh, which is executed with sh. This change keeps the script behavior intact while making the macOS detection logic POSIX-shell compatible.

• Scope: targeted shell compatibility fix

  • Replaced bash-specific prefix checks:
    • [[ "$OSTYPE" == "darwin"* ]]
  • With POSIX-safe parameter-expansion checks:
    • [ "${OSTYPE#darwin}" != "$OSTYPE" ]
  • Applied in both relevant branches of the script’s OS detection logic.

• Behavioral intent

  • Preserve existing branch selection and variable assignment flow.
  • Avoid shell parsing/runtime failures under dash/POSIX sh in install-test environments.

• Representative diff

  -if [[ "$OSTYPE" == "darwin"* ]] || [ -n "$PYTHON_VERSION" ];
  +if [ "${OSTYPE#darwin}" != "$OSTYPE" ] || [ -n "$PYTHON_VERSION" ];
  ...
  -    if [[ "$OSTYPE" == "darwin"* ]]; then
  +    if [ "${OSTYPE#darwin}" != "$OSTYPE" ]; then