[llvm] ci: set minimal permission on github workflow (PR #69737)

[Louis Dionne via llvm-commits]

ldionne wrote:

Ahhh, I think what happened is that I copied the clang-format workflow, then saw it had

```
permissions:
  pull-requests: write
```

and simply removed it cause I didn't need it. My bad, but thanks for fixing!

https://github.com/llvm/llvm-project/pull/69737