Allow configuring provisioner ssh socket timeout via environment variable

[PSU3D0]

I found skypilot could fail due to timeout errors on slower connections (Starlink, high contention network). Added a defaults-preserving global environment variable so users don't need need to resort to per-cloud ssh_proxy_command: 'none' workarounds.

• Allow overriding the raw SSH socket probe timeout via SKYPILOT_SSH_SOCKET_CONNECT_TIMEOUT, keeping the 1 s default, and surface misconfigurations with clear errors.
• Document the new environment variable alongside the existing provision.ssh_timeout option.
• Add focused unit tests covering default, override, and invalid values for the new timeout path.

Tested (run the relevant ones):

• Code formatting: install pre-commit (auto-check on commit) or bash format.sh
• Any manual or new tests for this PR (please specify below)
  • PYTHONPATH=. uv run --no-project pytest tests/unit_tests/test_provisioner.py
• All smoke tests: /smoke-test (CI) or pytest tests/test_smoke.py (local)
• Relevant individual tests: /smoke-test -k test_name (CI) or pytest tests/test_smoke.py::test_name (local)
• Backward compatibility: /quicktest-core (CI) or pytest tests/smoke_tests/test_backward_compat.py (local)