fix: Prevent external AWS configuration from breaking our boto3 config

[jstlaurent]

Changelogs

• Explicitly set the region when instantiating a boto3 client, to prevent it from picking one up from some external AWS config

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Checklist:

• [ ]~ Was this PR discussed in an issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.~
• Add tests to cover the fixed bug(s) or the newly introduced feature(s) (if appropriate).
• Update the API documentation if a new function is added, or an existing one is deleted.
• Write concise and explanatory changelogs above.
• If possible, assign one of the following labels to the PR: feature, fix, chore, documentation or test (or ask a maintainer to do it for you).

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Boto3 can automatically pick up external configuration for AWS, from various places in the environment: env variables, config files in user directory, etc. This recently caused a problem where it picked up a
region from a user that was not compatible with the Cloudflare R2 bucket we use. Explicitly setting the region when the client is created will avoid this issue in the future.

[cwognum]

Thanks, @jstlaurent ! 🙏

[mercuryseries]

Thanks @jstlaurent !