actions: Update to step-security change-files action

This repo is provided by them with the commit prior to when it was
compromised. LLVM has switched to the same repo for now.

We probably want to find some better solution to this at some point.

.github/workflows/pr-code-format.yml

@@ -28,7 +28,7 @@ jobs:
 
       - name: Get changed files
         id: changed-files
-        uses: tj-actions/changed-files@v39
+        uses: step-security/changed-files@3dbe17c78367e7d60f00d78ae6781a35be47b4a1 # v45.0.1
         with:
           separator: ","
           skip_initial_fetch: true