ci: add workflow_dispatch for manual reruns

Allows maintainers to re-trigger the KinD e2e and downstream unit test
workflows directly from the GitHub Actions UI without needing a dummy
commit, which is useful for investigating flakes.

Author: Ankitsinghsisodya

.github/workflows/kind-e2e.yaml

@@ -3,6 +3,7 @@ name: KinD e2e tests
 on:
   pull_request:
     branches: [ 'main', 'master' ]
+  workflow_dispatch:
 
 defaults:
   run:

.github/workflows/knative-downstream.yaml

@@ -17,6 +17,7 @@ name: Downstream
 on:
   pull_request:
     branches: [ 'main', 'master' ]
+  workflow_dispatch:
 
 jobs: