Description of your changes

PRs from fork are not lint or tested properly with our current workflow. This PR change the trigger to make sure it runs on opened or synchronize (details below).

synchronize: Triggered when a pull request's head branch is updated. For example, when the head branch is updated from the base branch, when new commits are pushed to the head branch, or when the base branch is changed.
(cf. https://docs.github.com/en/developers/webhooks-and-events/webhooks/webhook-events-and-payloads#pull_request)

How to verify this change

check tests on this PR: https://github.com/devfest-bugbust/aws-lambda-powertools-typescript/pull/3 (don't look at the changes, what matters are the actions run).

PR status

Is this ready for review?: YES
Is it a breaking change?: NO

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• The code coverage hasn't decreased
• I have added tests that prove my change is effective and works
• New and existing unit tests pass locally and in Github Actions
• Any dependent changes have been merged and published in downstream module
• The PR title follows the conventional commit semantics

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