Ensure PR preview deletion is working correctly in all cases

[gwarf]

As documented in #692, the PR preview deletion worked well when removing a label set on a PR created in a feature branch of this repo: PR #664 with https://github.com/EGI-Federation/documentation/actions/runs/12688246198/job/35364361677, but failed when a PR created in a branch from a fork was merged: PR $685 with https://github.com/EGI-Federation/documentation/actions/runs/12688986914/job/35366700297

Apparently the exact same workflow was used, but looking at the actions, the permissions where not the same (cf captures at #692 (comment)):

vs

[gwarf]

So looking at https://docs.github.com/en/actions/security-for-github-actions/security-guides/automatic-token-authentication:

You can use the permissions key to add and remove read permissions for forked repositories, but typically you can't grant write access.

[gwarf]

So maybe we have to use a similar logic as what we do for deploying the PR, with the deploy workflow that is triggered when the build pull request has been completed:

documentation/.github/workflows/deploy_pr_preview.yml

Lines 10 to 14 in 4a9a15b

	on:
	# https://docs.github.com/en/actions/reference/events-that-trigger-workflows#workflow_run
	workflow_run:
	workflows: ["Build pull request preview"]
	types: [completed]

[gwarf]

Or maybe we should just use https://docs.github.com/en/actions/writing-workflows/choosing-when-your-workflow-runs/events-that-trigger-workflows#pull_request_target