[llvm] ci: Fix `pr-code-format` permissions for private forks (PR #120838)

[via llvm-commits]

whisperity wrote:

I'll come back to this once we update our fork to the latest LLVM snapshot and see if there are still issues with it. So far I've realised I am a bit lost in the full diff between what we have (what existed in LLVM 18.1) and what exists in the upstream right now.

I would apologise for the noise, and will make sure to check the larger picture in the diffs next time it happens — I made the wrong assumption that the CI scripts would not be so constantly evolving. 😇 

https://github.com/llvm/llvm-project/pull/120838