[CI] Fix E2E workflows on PR trigger

By default 'issue_comment' trigger gets the context of a default branch.
If we want to test the actual PR, on which we created/edited a comment
we have to fetch and checkout a special ref for PR's merge commit.

Note, that 'pull/<no>/merge' ref is available as long as the PR
is not merged. This commit do not take this under consideration, since
this case should be fairly uncommon and would obfuscate the workflow.

Author: lukaszstolarczuk

.github/workflows/e2e_core.yml

@@ -66,10 +66,6 @@ jobs:
         ls -la ./
         rm -rf ./* || true
 
-    - uses: xt0rted/pull-request-comment-branch@d97294d304604fa98a2600a6e2f916a84b596dc7 # v2.0.0
-      id: comment-branch
-      if: ${{ always() && inputs.trigger != 'schedule' }}
-
     - name: Add comment to PR
       uses: actions/github-script@60a0d83039c74a4aee543508d2ffcb1c3799cdea # v7.0.1
       if: ${{ always() && inputs.trigger != 'schedule' }}
@@ -90,7 +86,18 @@ jobs:
       uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1
       with:
         path: ur-repo
-        ref: ${{ steps.comment-branch.outputs.head_ref }}
+
+    # On issue_comment trigger (for PRs) we need to fetch special ref for
+    # proper PR's merge commit. Note, this ref may be absent if the PR is already merged.
+    - name: Fetch PR's merge commit
+      if: ${{ inputs.trigger != 'schedule' }}
+      working-directory: ${{github.workspace}}/ur-repo
+      env:
+        PR_NO: ${{github.event.issue.number}}
+      run: |
+        git fetch -- https://github.com/${{github.repository}} +refs/pull/${PR_NO}/*:refs/remotes/origin/pr/${PR_NO}/*
+        git checkout origin/pr/${PR_NO}/merge
+        git rev-parse origin/pr/${PR_NO}/merge
 
     - name: Checkout SYCL
       uses: actions/checkout@b4ffde65f46336ab88eb53be808477a3936bae11 # v4.1.1