fix: add cleanup step for pkgdown pull request workflow

[akselthomsen]

Summary

Add step to restore original checkout for cleanup in PRs due to the post job cleanup being confused
when the workflow is running for a PR. The culprit here is the checkout of the gh-pages branch, and seems to only be a problem for packages with a custom setup action such as whirl.

Failing run on whirl:
https://github.com/NovoNordisk-OpenSource/whirl/actions/runs/20995252298/job/60350301124

New successful run using this branch:
https://github.com/NovoNordisk-OpenSource/whirl/actions/runs/20996573128/job/60354827527

Changes Made

• Added checkout step again.

Testing

• Everything passes and use tested in whirl above.

Checklist

• Code changes have been tested
• Documentation has been updated, if applicable
• All automated tests pass
• Coding style and naming conventions have been followed
• The PR is ready for review and merge

[github-actions]

Code coverage

Name	Coverage (%)
r.workflows	100
R/hello.R	100

[github-actions]

Github pages

Review the pkgdown webpage for the PR here

[vladimirobucina]

if it works, it works!