Prevent GitLab e2e tests from running twice on PRs

The tests are already triggered by the `pull-request` event (which is
also emitted when the head branch of the PR is updated). For PRs
created from branches of the ggp repo, having the `push` event for all
branches in the workflow definition will trigger it twice. Therefore I
changed it to only run on pushes to `main` now, following what we do
in other repos.

Signed-off-by: Max Jonas Werner <mail@makk.es>

.github/workflows/e2e-gitlab.yaml

@@ -5,7 +5,8 @@ name: e2e-gitlab
   workflow_dispatch:
   pull_request:
   push:
-    branches: ["*"]
+    branches:
+      - main
     tags-ignore: ["*"]
 
 jobs: