Avoid confusing morph and vulnix errors

ci-tools/vulnerability-scan

@@ -26,7 +26,8 @@ if morph_result=$(morph build morph/grid/testing/grid.nix 2>&1); then
   ln -s "$object" scan-target
 else
   echo "$morph_result"
-  exit 1
+  # Signal to the GitLab CI configuration this is a non-recoverable failure.
+  exit 4
 fi
 '