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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

Also, running the LLVM workflows in forks is not really a well-supported use case currently.

People hacking on the actions typically make sure they work in the monorepo at head and leave it at that. If people want to maintain this and the changes are simple enough and don't impact how they operate in the monorepo, I'm probably fine with them going upstream.

This patch significantly increases the permissions scope for this workflow and thus does impact how things operate in the monorepo, which I'm not a big fan of.

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