fix: fetch PR head before merging it in Check Merge Fast-Forward Only action

[UdjinM6]

Issue being fixed or feature implemented

v Run git fetch origin master:master
  git fetch origin master:master
  git checkout master
  if [[ "pull_request_target" == "pull_request"* ]]; then
    git merge --ff-only b216a402cc58cb153bebe137064fd0c804752d21
  else
    git merge --ff-only 3a18f087bf16fc7530b2b139a59a056320632b1f
  fi
  shell: /usr/bin/bash -e {0}
From https://github.com/dashpay/dash
 * [new branch]          master     -> master
Switched to branch 'master'
merge: b216a402cc58cb153bebe137064fd0c804752d21 - not something we can merge
Error: Process completed with exit code 1.

https://github.com/dashpay/dash/actions/runs/11492009432/job/31999602262?pr=6346

So it's picking the right commit sha now but it has no commit itself.

#6344 follow-up

What was done?

How Has This Been Tested?

Breaking Changes

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone (for repository code-owners and collaborators only)

[github-actions]

This pull request has conflicts, please rebase.

[PastaPastaPasta]

utACK c3aae9e