[llvm] [Github] Force files changed step in docs action to use GH API (PR #69763)

[Tobias Hieta via llvm-commits]

tru wrote:

One thing that changed files also solved for us here is that it made sure that the git history was there for git-clang-format. I want you to test this when there is a big commit list between the PR head and the base commit. I would expect this to fail since git-clang-format won't be able to compute the diff. 

https://github.com/llvm/llvm-project/pull/69763