Fix merging artifact chunks error when minio storage basepath is set (#28555) (#28568)
Backport #28555 by @fuxiaohei
Related to https://github.com/go-gitea/gitea/issues/28279
When merging artifact chunks, it lists chunks from storage. When storage
is minio, chunk's path contains `MINIO_BASE_PATH` that makes merging
break.
<del>So trim the `MINIO_BASE_PATH` when handle chunks.</del>
Update the chunk file's basename to retain necessary information. It
ensures that the directory in the chunk's path remains unaffected.
Co-authored-by: FuXiaoHei <fuxiaohei@vip.qq.com>
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