Use multiple accounts in canary workflow

[sobolk]

Problem

Canary workflow doesn't use multiple e2e accounts yet.

Previous PR that introduced multiple accounts to health checks #1879 .

Changes

1. Create action to encapsulate account selection logic.
2. Use it canaries in addition to health checks.

Validation

1. This PR checks
2. Canary run https://github.com/aws-amplify/amplify-backend/actions/runs/10497624775

Checklist

• If this PR includes a functional change to the runtime behavior of the code, I have added or updated automated test coverage for this change.
• If this PR requires a change to the Project Architecture README, I have included that update in this PR.
• If this PR requires a docs update, I have linked to that docs PR above.
• If this PR modifies E2E tests, makes changes to resource provisioning, or makes SDK calls, I have run the PR checks with the run-e2e label set.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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[Amplifiyer]

Will this be the place to see which account is being used?

Also are we randomizing the region as well? In case same account is given to multiple test runners?

[sobolk]

Will this be the place to see which account is being used?

That's one of the places. The other (already existing) is deployment log in the job logs (there's account ID in ARNs that are printed there).

Also are we randomizing the region as well? In case same account is given to multiple test runners?

We do not. For canaries it wouldn't make much sense as randomizing [random_account, region] is already achieving uniform distribution across both dimensions.

This is however a good idea to explore for health checks. But I'd rather do that in separate PR.

Also keep in mind that our objective in canary job is to run checks in all regions in the matrix. In health checks spreading load across randomly selected region would help with avoiding throttling with less accounts.