fix(ci): Unblock external-contributors checkout on fork PR merges

[Lms24]

The external-contributors workflow started failing after #21898 bumped actions/checkout to v7. That release blocks checkout in pull_request_target workflows when the triggering PR came from a fork, even when only checking out the base branch.

This PR ensures we explicitly check out develop and set allow-unsafe-pr-checkout: true.
The workflow never uses fork PR code. It only reads PR metadata from the
event payload and updates CHANGELOG.md on develop.

Broken run: https://github.com/getsentry/sentry-javascript/actions/runs/29263604917

[sentry]

Bug: The workflow is missing allow-unsafe-pr-checkout: true in the checkout step, which was explicitly mentioned as part of the fix in the PR description.
_{Severity: MEDIUM}

Suggested Fix

Add allow-unsafe-pr-checkout: true to the actions/checkout step in the workflow file. This aligns the code with the original intent described in the pull request and ensures the checkout step does not fail due to security restrictions in pull_request_target workflows.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: .github/workflows/external-contributors.yml#L24-L26

Potential issue: The `pull_request_target` workflow was updated to use
`actions/checkout@v7` and explicitly check out the `develop` branch. However, the change
omits `allow-unsafe-pr-checkout: true`, which was stated as necessary in the pull
request description. While checking out the base branch `ref: develop` might circumvent
the new security restrictions in `actions/checkout@v7`, the omission contradicts the
stated intent of the fix, which was prompted by a "broken run". This discrepancy
suggests the fix is incomplete and the workflow might still fail to check out code under
certain conditions.

_{Did we get this right? 👍 / 👎 to inform future reviews.}