exec: Abort on error

We need exit on the first error, as otherwise we cannot track a bad
execution when no exec is done (when post is not empty).

garlic/stages/exec.nix

@@ -25,7 +25,7 @@ stdenv.mkDerivation {
   phases = [ "installPhase" ];
   installPhase = ''
     cat > $out <<'EOF'
-    #!/bin/sh
+    #!/bin/sh -e
     ${env}
 
     ''+pre+''