Make ascn's need optional

If changes did not trigger a klarlack build, gitlab CI would file this
complaint:

  Found errors in your .gitlab-ci.yml:

    'build:ascn' job needs 'build:klarlack' job
    but 'build:klarlack' is not in any previous stage

despite the fact that the ci file lints just fine.

IIUC, the issue is that for the triggered pipeline, the klarlack build job
just would not exist.

In this case, we can build ascn on top of the previous klarlack build.

.gitlab-ci.yml

@@ -92,7 +92,9 @@ build:klarlack:manual:
 # ascn: same as klarlack, plus Dockerfile.ascn
 build:ascn:
   extends: .build-ascn
-  needs: ["build:klarlack"]
+  needs:
+    - job: build:klarlack
+      optional: true
   rules:
     - changes:
       - container/Dockerfile.ascn