ci(workflows): specify pull_request_target branches

[caugner]

Description

Update all workflows using pull_request_target to specify trusted target branches, such as main.

Motivation

Security best practice to limit pull_request_target workflows to trusted branches, reducing the attack surface for malicious pull requests.

The pull_request_target trigger causes the workflow to run in the context of the base branch with access to secrets, so it's important:

• to specify only trusted branches, to avoid that the workflow runs on an untrusted branch,
• to specify all trusted branches, to avoid that the workflow is skipped on PRs targeting a trusted branch.

Additional details

See: https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target

Related issues and pull requests

Part of mdn/fred#1021.