GHA: Update testing-farm action to use main branch

[lsm5]

v2 was released quite sometime ago.
GHA: set trigger to pull_request_target

• Remove checkout step (Testing Farm clones the repo on VM)
• Use PR head repo/sha to test actual PR changes
• Fallback to base repo for push events
• Keeps secrets isolated from untrusted PR code

Summary by Sourcery

Update the Testing Farm XML validation workflow to run on pull_request_target and use PR-specific sources while keeping secrets isolated from untrusted code.

CI:

• Switch the XML validation workflow trigger from pull_request to pull_request_target to improve security for PR runs.
• Update the Testing Farm GitHub Action to track the main branch and use PR head repository/commit with a fallback to the base repository for non-PR events.
• Remove the explicit checkout step from the XML validation workflow since the Testing Farm action clones the repository itself.

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Updates the XML validation GitHub Actions workflow to run on pull_request_target, use the main branch of the Testing Farm action, drop the checkout step, and configure Testing Farm to test the actual PR head for pull requests while falling back to the base repo/ref for other events.

File-Level Changes

Change	Details	Files
Switch workflow trigger from pull_request to pull_request_target to improve secret isolation for PRs.	Replace the pull_request event trigger with pull_request_target for the workflow while keeping push to main unchanged Ensure that the workflow still only runs for the main branch	.github/workflows/xmllint-validation.yml
Update Testing Farm action usage and inputs to rely on the PR head repo/ref and remove unnecessary checkout.	Remove the explicit checkout step since Testing Farm clones the repository itself Change the Testing Farm GitHub Action reference from version v2 to the main branch Set git_url to the PR head repository clone_url with a fallback to the base repository URL for non-PR events Set git_ref to the PR head SHA with a fallback to github.ref for non-PR events	.github/workflows/xmllint-validation.yml

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