srun: Add SLURM_JOB_NODELIST env var

This makes srun more consistent with salloc and sbatch

src/common/env.c

@@ -1319,6 +1319,8 @@ env_array_for_step(char ***dest,
 	env_array_overwrite_fmt(dest, "SLURM_STEP_ID", "%u", step->job_step_id);
 	env_array_overwrite_fmt(dest, "SLURM_STEP_NODELIST",
 				"%s", step->step_layout->node_list);
+	env_array_append_fmt(dest, "SLURM_JOB_NODELIST",
+			     "%s", step->step_layout->node_list);
 	if (cluster_flags & CLUSTER_FLAG_BG) {
 		char geo_char[HIGHEST_DIMENSIONS+1];