fix: generate previews from forked repos

[xiaolou86]

Description of the Changes

fix: Generate previews from forked repos

Closes #1043

Description of the Solution

1. Change the event from pull_request to pull_request_target. permissions-for-the-github_token

2. Change the branch ref when checkout. Because its event (pull_request_target) runs in the context of the base of the pull request, rather than in the context of the merge commit as the pull_request event does. base branch

3. Since of the above 2, the workflow triggered from pull_request_target event will run only after the .yml config has been merged into repo.

PR Preview URL

After you push a commit to this PR, a preview is built and a URL to the root of the preview appears in the comment feed.

Paste here the specific URL(s) of the content that this PR addresses.

Check List

• Changes have been done against main branch, and PR does not conflict
• PR title follows the convention: <docs/feat/fix/chore>(optional scope): <description>, e.g: fix: minor typos in code

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This change is 

[xiaolou86]

@stoobie I have finished, please help me to review.

[xiaolou86]

The workflow triggered from pull_request_target event won't run until this .yml config has been merged into repo.
After that, the workflow will run when other PRs are created.

[stoobie]

@xiaolou86 Wonderful! Thank you for your hard work on this! I need to have another set of eyes review the code before I can merge it.

[guyvelleman]

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @xiaolou86)

[stoobie]

@guyvelleman gave an LGTM offline.