[releng] Issue #6832: Dont sign the checksums if they are already signed - stage-release.sh

[Mohan Boddu]

mohanboddu reported a new issue against the project: `releng` that you are following:
``
Sometimes during the staging of GOLD RC compose using stage-release.sh, we need to rerun
the script if rsync skips any of the files. But when we re-run the scripts, its resigning
the checksums again, so its better to add a check if the checksums are already signed then
skip them by saying that they were already signed.
``

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https://pagure.io/releng/issue/6832