Propagate fuzz_targets for RemoteRegularBuild. (#4166)

This is required for OSS-Fuzz after the uworker refactor. This fixes the
following exception:

```
Error occurred while working on task.
Traceback (most recent call last):
  File "/mnt/scratch0/bots/oss-fuzz-linux-zone1-host-bsw9-13/clusterfuzz/src/python/bot/startup/run_bot.py", line 146, in task_loop
    commands.process_command(task)
  File "/mnt/scratch0/bots/oss-fuzz-linux-zone1-host-bsw9-13/clusterfuzz/src/clusterfuzz/_internal/bot/tasks/commands.py", line 249, in process_command
    task.high_end, task.is_command_override)
  File "/mnt/scratch0/bots/oss-fuzz-linux-zone1-host-bsw9-13/clusterfuzz/src/clusterfuzz/_internal/bot/tasks/commands.py", line 159, in wrapper
    return func(task_name, task_argument, job_name, *args, **kwargs)
  File "/mnt/scratch0/bots/oss-fuzz-linux-zone1-host-bsw9-13/clusterfuzz/src/clusterfuzz/_internal/bot/tasks/commands.py", line 433, in process_command_impl
    preprocess)
  File "/mnt/scratch0/bots/oss-fuzz-linux-zone1-host-bsw9-13/clusterfuzz/src/clusterfuzz/_internal/bot/tasks/commands.py", line 218, in run_command
    result = task.execute(task_argument, job_name, uworker_env)
  File "/mnt/scratch0/bots/oss-fuzz-linux-zone1-host-bsw9-13/clusterfuzz/src/clusterfuzz/_internal/bot/tasks/task_types.py", line 105, in execute
    self.execute_locally(task_argument, job_type, uworker_env)
  File "/mnt/scratch0/bots/oss-fuzz-linux-zone1-host-bsw9-13/clusterfuzz/src/clusterfuzz/_internal/bot/tasks/task_types.py", line 63, in execute_locally
    uworker_output = utasks.uworker_main_no_io(self.module, uworker_input)
  File "/mnt/scratch0/bots/oss-fuzz-linux-zone1-host-bsw9-13/clusterfuzz/src/clusterfuzz/_internal/bot/tasks/utasks/__init__.py", line 211, in uworker_main_no_io
    uworker_output = utask_module.utask_main(uworker_input)
  File "/mnt/scratch0/bots/oss-fuzz-linux-zone1-host-bsw9-13/clusterfuzz/src/clusterfuzz/_internal/bot/tasks/utasks/fuzz_task.py", line 1919, in utask_main
    return session.run()
  File "/mnt/scratch0/bots/oss-fuzz-linux-zone1-host-bsw9-13/clusterfuzz/src/clusterfuzz/_internal/bot/tasks/utasks/fuzz_task.py", line 1731, in run
    self.fuzz_task_output.fuzz_targets.extend(build_setup_result.fuzz_targets)
TypeError: Value must be iterable

```

Author: oliverchang

src/clusterfuzz/_internal/bot/untrusted_runner/build_setup.py

@@ -20,7 +20,7 @@
 # pylint: disable=no-member
 
 
-def _build_response(result):
+def _build_response(build, result):
   if not result:
     return untrusted_runner_pb2.SetupBuildResponse(result=False)
 
@@ -32,12 +32,13 @@ def _build_response(result):
       build_dir=environment.get_value('BUILD_DIR'),
       build_url=environment.get_value('BUILD_URL'),
       fuzz_target=environment.get_value('FUZZ_TARGET'),
-      fuzz_target_count=environment.get_value('FUZZ_TARGET_COUNT'))
+      fuzz_target_count=environment.get_value('FUZZ_TARGET_COUNT'),
+      fuzz_targets=build.fuzz_targets or [])
 
 
 def setup_regular_build(request):
   """Set up a regular build."""
   build = build_manager.RegularBuild(request.base_build_dir, request.revision,
                                      request.build_url, request.target_weights,
                                      request.build_prefix)
-  return _build_response(build.setup())
+  return _build_response(build, build.setup())

src/clusterfuzz/_internal/bot/untrusted_runner/build_setup_host.py

@@ -45,6 +45,8 @@ def _handle_response(build, response):
       build_manager.set_environment_vars([fuzzer_directory])
 
   environment.set_value('APP_REVISION', build.revision)
+  build.fuzz_targets = list(response.fuzz_targets)
+
   return True
 
 

src/clusterfuzz/_internal/protos/untrusted_runner.proto

@@ -46,6 +46,7 @@ message SetupBuildResponse {
   optional string build_url = 6;
   optional string fuzz_target = 7;
   optional int32 fuzz_target_count = 8;
+  repeated string fuzz_targets = 9;
 }
 
 message SetupRegularBuildRequest {