ci: fix git unshallow opts on main

[jedevc]

https://github.com/dagger/dagger/actions/runs/11058080873/job/30723306886

We need --update-head-ok here:

-u
--update-head-ok

By default git fetch refuses to update the head which corresponds to the current branch. This flag disables the check. This is purely for the internal use for git pull to communicate with git fetch, and unless you are implementing your own Porcelain you are not supposed to use it.

---

Wrong! There's a reason it tells you not to use this flag, we can instead avoid this by just entering a detached head state intentionally, then we can rewrite all the refs we like, no worries.