fix(ci): use the right sha in release PRs (#11365)

## Problem

`github.sha` contains a merge commit of `head` and `base` if we're in a
PR. In release PRs, this makes no sense, because we fast-forward the
`base` branch to contain the changes from `head`.

Even though we correctly use `${{ github.event.pull_request.head.sha ||
github.sha }}` to reference the git commit when building artifacts, we
don't use that when checking out code, because we want to test the merge
of head and base usually. In the case of release PRs, we definitely
always want to test on the head sha though, because we're going to
forward that, and it already has the base sha as a parent, so the merge
would end up with the same tree anyway.

As a side effect, not checking out `${{
github.event.pull_request.head.sha || github.sha }}` also caused
https://github.com/neondatabase/neon/actions/runs/13986389780/job/39173256184#step:6:49
to say `release-tag=release-compute-8187`, while
https://github.com/neondatabase/neon/actions/runs/14084613121/job/39445314780#step:6:48
is talking about `build-tag=release-compute-8186`

## Summary of changes
Run a few things on `github.event.pull_request.head.sha`, if we're in a
release PR.

.github/workflows/trigger-e2e-tests.yml

@@ -9,6 +9,9 @@ on:
       github-event-name:
         type: string
         required: true
+      github-event-json:
+        type: string
+        required: true
 
 defaults:
   run:
@@ -48,6 +51,7 @@ jobs:
     uses: ./.github/workflows/_meta.yml
     with:
       github-event-name: ${{ inputs.github-event-name || github.event_name }}
+      github-event-json: ${{ inputs.github-event-json || toJSON(github.event) }}
 
   trigger-e2e-tests:
     needs: [ meta ]