post-process-oabi: Flush tempfile before using it

The NamedTemporaryFile is buffered by default, so there might be data
left in the buffer when shutil.copyfileobj() completes. This would then
result in TarFile.extractall() failing due to trying to read from an
incomplete archive.

post-process-oabi.py

@@ -35,6 +35,7 @@ def main():
     with urllib.request.urlopen(arm64e_sdk_url) as response, \
             tempfile.NamedTemporaryFile(suffix=".tar.xz") as archive:
         shutil.copyfileobj(response, archive)
+        archive.flush()
         arm64e_artifact_path = Path(archive.name)
 
         with tempfile.TemporaryDirectory() as patched_artifact_dir: