pre-commit/check-merge-conflicts-2: fix use outside dev shell (backport #12373)

[mergify]

Note that this is just a script that is meant to run outside a derivation (but also can be called by a derivation builder). touch $out does not belong in it.

touch $out worked accidentally in the derivation-based check, and also in the dev shell, but if pre-commit is invoked without the dev shell it would fail.

Motivation

Context

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This is an automatic backport of pull request #12373 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 77a8386 has failed:

On branch mergify/bp/2.24-maintenance/pr-12373
Your branch is ahead of 'origin/2.24-maintenance' by 1 commit.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 77a838602.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   tests/functional/git-hashing/simple.sh

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)
	deleted by us:   tests/functional/git-hashing/fixed.sh

Cherry-pick of 414c346 has failed:

On branch mergify/bp/2.24-maintenance/pr-12373
Your branch is ahead of 'origin/2.24-maintenance' by 2 commits.
  (use "git push" to publish your local commits)

You are currently cherry-picking commit 414c34656.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   tests/functional/help.sh

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally