Fix srun availability check (#15211)

command -v

Co-authored-by: Justus Schock <12886177+justusschock@users.noreply.github.com>

src/lightning_lite/plugins/environments/slurm.py

@@ -160,7 +160,10 @@ def _validate_srun_used() -> None:
         """
         if _IS_WINDOWS:
             return
-        srun_exists = subprocess.call(["which", "srun"]) == 0
+        try:
+            srun_exists = subprocess.call(["command", "-v", "srun"]) == 0
+        except FileNotFoundError:
+            srun_exists = False
         if srun_exists and not _is_srun_used():
             hint = " ".join(["srun", os.path.basename(sys.executable), *sys.argv])[:64]
             rank_zero_warn(