stages: add baywatch stage to check the exit code

This workaround stage prevents srun from returning 0 to the upper stages
when a signal happens after MPI_Finalize. It writes the return code to a
file named .srun.rc.$rank and later checks that exists and contains a 0.

When the program is killed, exits with non-zero and the error is
propagated to the baywatch stage, which aborts immediately without
creating the rc file.

garlic/stdexp.nix

@@ -99,13 +99,17 @@ rec {
         inherit nextStage;
       }
     );
+
+    baywatch = {nextStage, ...}: stages.baywatch {
+      inherit nextStage;
+    };
   };
 
   stdPipelineOverride = {overrides ? {}}:
   let
     stages = stdStages // overrides;
   in
-    with stages; [ sbatch isolate control srun isolate ];
+    with stages; [ sbatch isolate control srun isolate baywatch ];
 
 
   stdPipeline = stdPipelineOverride {};