improve PR workflow

[maelle]

• Now preview steps are cleanly skipped for forks. Example in another repo.

• There's no longer one comment per commit. Advantage: less spamming. Downside: the preview URL is now three clicks away, not one.

1. Click on "Show all checks".
2. Click on "details" for the second check (check that's called "Preview book)
3. There, click on the preview link.

I tried adding the preview URL as details URL like what Netlify does for Hugo websites but I can't. I suppose it's because I don't host the app used to create the check (that app being GitHub actions).

[mpadge]

--- perfect, thank you!

[maelle]

Thank you for making me learn Bash basics :sweat_smile: