fix: ensure systemd service is restarted if nvidia-smi fails

We run nvidia-smi in an ExecStart statement instead of an ExecCondition statement
so that the unit gets restarted in case of failures. With ExecCondition, if the
command returns an error code between 1 and 254 (inclusive) the remaining commands
are skipped and the unit is not marked as failed, and thus the service is not
restarted.

Signed-off-by: Christopher Desiniotis <cdesiniotis@nvidia.com>