fix systemd unit conditions to work on WSL2

On WSL2, nvidia-smi lives at /usr/lib/wsl/lib/nvidia-smi rather than
/usr/bin or /usr/sbin, and the NVIDIA kernel modules (nvidia.ko /
nvidia-current.ko) are not present since GPU access goes through the
DirectX Graphics infrastructure (/dev/dxg).

Add /usr/lib/wsl/lib/nvidia-smi as a third OR ConditionPathExists and
extend ExecCondition to succeed when /dev/dxg exists so the service
runs on WSL2 without breaking the check on standard driver installs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Signed-off-by: Evan Lezar <elezar@nvidia.com>