ci: Use pull_request_target in update label workflow

When the workflow is triggered from a PR opened in a fork, it fails with
the following message:

`GraphQL: Resource not accessible by integration (removeLabelsFromLabelable)`

To fix this, we need to run the workflow in the context of the pull
request base, so the commit changes the event to be
`pull_request_target` instead of `pull_request`.

Signed-off-by: Fabio Falzoi <fabio.falzoi@isovalent.com>

.github/workflows/call-backport-label-updater.yaml

@@ -1,7 +1,7 @@
 ---
     name: Call Backport Label Updater
     on:
-      pull_request:
+      pull_request_target:
         types:
           - closed
         branches: