[3.12] gh-133410: Fix PR detection in build workflow (GH-133671) (#134057)

Co-authored-by: Hugo van Kemenade <[email protected]>

Author: miss-islington

.github/workflows/reusable-context.yml

@@ -97,6 +97,7 @@ jobs:
       run: python Tools/build/compute-changes.py
       env:
         GITHUB_DEFAULT_BRANCH: ${{ github.event.repository.default_branch }}
+        GITHUB_EVENT_NAME: ${{ github.event_name }}
         CCF_TARGET_REF: ${{ github.base_ref || github.event.repository.default_branch }}
         CCF_HEAD_REF: ${{ github.event.pull_request.head.sha || github.sha }}
 

Tools/build/compute-changes.py

@@ -57,7 +57,7 @@ class Outputs:
 
 def compute_changes() -> None:
     target_branch, head_ref = git_refs()
-    if target_branch and head_ref:
+    if os.environ.get("GITHUB_EVENT_NAME", "") == "pull_request":
         # Getting changed files only makes sense on a pull request
         files = get_changed_files(target_branch, head_ref)
         outputs = process_changed_files(files)