…racer-Cloud#1099)

The post-merge ``test (windows-latest)`` job on commit 67dc64d (PR Tracer-Cloud#1090
landing) is failing with two distinct symptoms in the install.sh test
suites added by PR Tracer-Cloud#1064:

1. ``tests/cli/test_install_sh_path.py`` — 12 tests fail because the
   subprocess call ``subprocess.run(["bash", "-c", script])`` resolves
   ``bash`` to ``wsl.exe`` on the GitHub Actions ``windows-latest``
   runner, and that runner has no installed WSL distribution. Every
   ``_run`` call exits 1 with a literal "Windows Subsystem for Linux
   has no installed distributions" message, so none of the asserted
   rc files (``.zshrc``/``.bashrc``/``config.fish``) ever get written.

2. ``tests/cli/test_install_sh_resolution.py`` — module fails to import
   on Windows: the existing
   ``@pytest.mark.skipif(os.geteuid() == 0, reason=...)`` decorator
   evaluates ``os.geteuid()`` at decorator-application time, but
   ``os.geteuid`` does not exist on Windows. ``AttributeError`` fires
   *before* pytest can read any markers, blocking collection of every
   test in the module.

Both files exclusively test ``install.sh``, which is a POSIX shell
script (zsh/bash/fish RC files) with no Windows analogue, so the
correct fix for the Windows runner is to skip them.

Changes:

- ``test_install_sh_path.py``: module-level
  ``pytestmark = pytest.mark.skipif(sys.platform == "win32", ...)``
  with a comment that names the failure mode.

- ``test_install_sh_resolution.py``: same module-level skipif, *and*
  defensively replace ``os.geteuid() == 0`` with
  ``_RUNNING_AS_ROOT = hasattr(os, "geteuid") and os.geteuid() == 0``
  evaluated once at module load. ``hasattr`` short-circuits the ``and``
  on Windows so ``os.geteuid()`` is never called there — the module
  imports cleanly even before pytest reads ``pytestmark``.

No source-checkout behaviour change: skipif is a no-op on macOS/Linux,
and the existing root-only test still skips correctly on those
platforms (verified by running ``pytest tests/cli/test_install_sh_path.py
tests/cli/test_install_sh_resolution.py`` locally — 15 / 15 pass).
Full unit suite: 4164 pass / 2 skipped / 1 xfailed.

Closes Tracer-Cloud#1099.