fix(backups/_MixinBackupWriter#afterBackup): execute merge on S3

[fbeauchamp]

Check list

Check if done, if not relevant leave unchecked.

• PR reference the relevant issue (e.g. Fixes #007 or See xoa-support#42)
• if UI changes, a screenshot has been added to the PR
• documentation updated
• CHANGELOG.unreleased.md:
  • enhancement/bug fix entry added
  • list of packages to release updated (${name} v${new version})
• I have tested added/updated features (and impacted code)
  • unit tests (e.g. cron/parse.spec.js)
  • if xo-server API changes, the corresponding test has been added to/updated on xo-server-test
  • at least manual testing

Process

1. create a PR as soon as possible
2. mark it as WiP: (Work in Progress) if not ready to be merged
3. when you want a review, add a reviewer (and only one)
4. if necessary, update your PR, and re- add a reviewer

From the Four Agreements:

1. Be impeccable with your word.
2. Don't take anything personally.
3. Don't make assumptions.
4. Always do your best.

[julien-f]

Format is weird, the PR mention should be at the end.

Any refs for this PR? (issues, forum, etc)

[fbeauchamp]

that's from https://xcp-ng.org/forum/topic/5256/s3-backup-try-it/119 , but I don't know how to target a specific post

[olivierlambert]

Right-click on the post time to get the URL of the specific post