presubmit: Run build file check from root

Running the 'source_is_in_build_files' step in presubmit only succeeds
if the user runs it from the root directory. Change the step to run from
root regardless of the current working directory.

Change-Id: I929d5cfcdfcaafc259bc20dc34052c2d5adfb7bf
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/34520
Reviewed-by: Rob Mohr <mohrr@google.com>
Commit-Queue: Prashanth Swaminathan <prashanthsw@google.com>
diff --git a/pw_presubmit/py/pw_presubmit/pigweed_presubmit.py b/pw_presubmit/py/pw_presubmit/pigweed_presubmit.py
index cade0aa..9e8b034 100755
--- a/pw_presubmit/py/pw_presubmit/pigweed_presubmit.py
+++ b/pw_presubmit/py/pw_presubmit/pigweed_presubmit.py
@@ -390,8 +390,8 @@
         _GN_SOURCES_IN_BUILD,
         ctx.paths,
         bazel_dirs=[ctx.root],
-        gn_build_files=git_repo.list_files(
-            pathspecs=['BUILD.gn', '*BUILD.gn']))
+        gn_build_files=git_repo.list_files(pathspecs=['BUILD.gn', '*BUILD.gn'],
+                                           repo_path=ctx.root))
 
     if missing:
         _LOG.warning(