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MINOR: [CI] Use secrets for bucket name in preview-docs job (#14270)

For technical reasons we have to change the s3 bucket, to avoid having to make changes to the workflow in the future I have changed the envvar so they use secrets.

SSL on bucket hosted static sites is non-trivial and not really necessary (it's a static site after all). 

Here a render with the changes:
```yml
      - name: Upload preview to S3
        env:
          AWS_ACCESS_KEY_ID: ${{ secrets.CROSSBOW_DOCS_AWS_ACCESS_KEY_ID }}
          AWS_SECRET_ACCESS_KEY: ${{ secrets.CROSSBOW_DOCS_AWS_SECRET_ACCESS_KEY }}
          AWS_DEFAULT_REGION: ${{ secrets.CROSSBOW_DOCS_S3_BUCKET_REGION }} 
          BUCKET: ${{ secrets.CROSSBOW_DOCS_S3_BUCKET }}
        run: |
          aws s3 cp build/docs/ $BUCKET/pr_docs/master/ --recursive
          echo ":open_book: You can find the preview here: http://crossbow.voltrondata.com/pr_docs/master" >> $GITHUB_STEP_SUMMARY
```


Authored-by: Jacob Wujciak-Jens <jacob@wujciak.de>
Signed-off-by: Antoine Pitrou <antoine@python.org>