Switch to pull_request_target trigger in workflow adding test list (#…

…616)

Despite having configured the workflow to trigger when a PR with base
branch `release` and head branch `stage-live` is opened, we didn't see
the workflow to trigger in those circumstances. We've tested similar
config in other repository and it was working there. The difference was
that in the other repository the PR opened wasn't showing warning about
the branch being out-of-date (and in case of the release PRs in `dapp`
we see that warning). So it looks that this may affect workflow
triggering.
In this PR we want to verify if changing the triggering event to
[`pull_request_target`](https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target)
will help. This event runs in the context of the base of the pull
request, not in the context of the merge commit.

.github/workflows/test-list.yml

@@ -6,7 +6,7 @@
 name: Add test list to release PRs
 
 on:
-  pull_request:
+  pull_request_target:
     types:
       - opened
     branches: