Backups: Two units creating a backup at the same time could overwrite the backup file

[reneradoi]

Backups can be created on all units of a Valkey deployment (functionality added in #59). If the create-backup action is run at the exact same time on multiple units, they will create a backup with the same filename / backup-id, and the file uploaded to S3 storage might be corrupt/overwritten.

For details or reproducability see the review comment: #59 (review)