Do not try to build the container images when PR is closed (#150)

At the time this job runs, the branch might have been deleted already,
causing unnecessary failures (+notifications for the PR owner) [1]:
```
A branch or tag with the name '$branch_name' could not be found
```

[1] https://github.com/janus-idp/operator/actions/workflows/pr-container-build.yaml?query=event%3Apull_request_target+is%3Afailure

.github/workflows/pr-container-build.yaml

@@ -21,7 +21,7 @@ on:
   # we are adding an 'authorize' job that checks if the workflow was triggered from a fork PR. In that case, the "external" environment
   # will prevent the job from running until it's approved manually by human intervention.
   pull_request_target:
-    types: [opened, synchronize, reopened, ready_for_review, closed]
+    types: [opened, synchronize, reopened, ready_for_review]
     branches: [ main ]
 
 concurrency: