CI(pre-merge-checks): do not run conclusion job for PRs (#10619)

## Problem

While working on https://github.com/neondatabase/neon/pull/10617 I
(unintentionally) merged the PR before the main CI pipeline has
finished.
I suspect this happens because we have received all the required job
results from the pre-merge-checks workflow, which runs on PRs that
include changes to relevant files.

## Summary of changes
- Skip the `conclusion` job in `pre-merge-checks` workflows for PRs

.github/workflows/pre-merge-checks.yml

@@ -95,7 +95,8 @@ jobs:
   # - conclusion
   # - neon-cloud-e2e
   conclusion:
-    if: always()
+    # Do not run job on Pull Requests as it interferes with the `conclusion` job from the `build_and_test` workflow
+    if: always() && github.event_name == 'merge_group'
     permissions:
       statuses: write # for `github.repos.createCommitStatus(...)`
       contents: write