Fix fuchsia upload symbols. (flutter#47938)

The fuchsia bucket does not have a delete ACL which is required to replace files that already exist. This change skips uploading the file if it already exists.

Bug: flutter/flutter#126461
Bug: flutter/flutter#135189

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

tools/fuchsia/upload_to_symbol_server.py

@@ -32,6 +32,19 @@ def remote_filename(exec_path):
   return ''.join(parts[-2:])
 
 
+def exists_remotely(remote_path):
+  gsutil = os.path.join(os.environ['DEPOT_TOOLS'], 'gsutil.py')
+  command = ['python3', gsutil, '--', 'stat', remote_path]
+  process = subprocess.Popen(
+      command, stderr=subprocess.PIPE, stdout=subprocess.PIPE
+  )
+  stdout, stderr = process.communicate()
+  return_code = process.wait()
+  if return_code == 0:
+    print('%s exists - skipping copy' % remote_path)
+  return return_code == 0
+
+
 def process_symbols(should_upload, symbol_dir):
   full_path = os.path.join(BUILD_ROOT_DIR, symbol_dir)
 
@@ -50,9 +63,9 @@ def process_symbols(should_upload, symbol_dir):
         FUCHSIA_ARTIFACTS_BUCKET_NAME, FUCHSIA_ARTIFACTS_DEBUG_NAMESPACE,
         remote_filename(file)
     )
-    if should_upload:
+    if should_upload and not exists_remotely(remote_path):
       gsutil = os.path.join(os.environ['DEPOT_TOOLS'], 'gsutil.py')
-      command = ['python3', gsutil, '--', 'cp', gsutil, file, remote_path]
+      command = ['python3', gsutil, '--', 'cp', file, remote_path]
       subprocess.check_call(command)
     else:
       print(remote_path)