ci: Harden labeler workflow, remove unnecessary checkout from pull_request_target job (#20637)

## Which issue does this PR close?

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- Closes #.

## Rationale for this change
This PR removes the checkout step from the labeler workflow and keeps
labeling behavior unchanged.

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## What changes are included in this PR?

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The workflow runs on `pull_request_target`, which has elevated repo
context. `actions/labeler` does not require a local checkout to work
with `configuration-path`; if the file is not on disk, it fetches it via
the GitHub API.

Removing checkout reduces attack surface and avoids exposing persisted
git credentials to subsequent steps.

## Are these changes tested?
Yes, tested on my forked. 

I force pushed this change to my fork's `main` branch, then open a [test
PR](kevinjqliu#2) against it. The
[labeler github action ran successfully on my
fork](https://github.com/kevinjqliu/datafusion/actions/runs/22553132113/job/65326120264)
and labeled the PR

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## Are there any user-facing changes?

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No

Author: kevinjqliu

.github/workflows/labeler.yml

@@ -39,8 +39,6 @@ jobs:
       contents: read
       pull-requests: write
     steps:
-      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2
-
       - name: Assign GitHub labels
         if: |
           github.event_name == 'pull_request_target' &&