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/index.nix

@@ -76,6 +76,7 @@
   stages = {
     sbatch     = callPackage ./stages/sbatch.nix { };
     srun       = callPackage ./stages/srun.nix { };
+    baywatch   = callPackage ./stages/baywatch.nix { };
     control    = callPackage ./stages/control.nix { };
     exec       = callPackage ./stages/exec.nix { };
     script     = callPackage ./stages/script.nix { };