Avoid deleting the workspace twice. (#7923)

certbot-ci/certbot_integration_tests/utils/acme_server.py

@@ -86,6 +86,7 @@ class ACMEServer(object):
                                                 'alpine', 'rm', '-rf', '/workspace/boulder'])
                 process.wait()
         finally:
+            if os.path.exists(self._workspace):
                 shutil.rmtree(self._workspace)
         if self._stdout != sys.stdout:
             self._stdout.close()