GitHub workflow missing ref PR merge

[oriy]

checkout fails checking out refs/pull/<PR_ID>/merge

Run actions/checkout@v4.2.2
  with:
    ref: refs/pull/<PR_ID>/merge

added workflow demonstrating the checkout failure - #1981

issue reproduces:

• create a PR
• checkout PR merge branch refs/pull/<PR_ID>/merge - ✅ successfully
• close PR, reopen PR
• checkout PR merge branch refs/pull/<PR_ID>/merge - ⛔ FAIL

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started writing this workflow only after trying v4.2.1,
expected #1924 to fix the regression,
however, regression still here