Fix whoami_async call in sync context (#7083)

This fixes a RuntimeWarning in tests/calculations/test_stash.py

  tests/conftest.py:199: RuntimeWarning: coroutine 'BlockingTransport.whoami_async' was never awaited
    return process.prepare_for_submission(folder)

Author: danielhollas

src/aiida/calculations/unstash.py

@@ -109,7 +109,7 @@ def traverse(node_):
             else:  # UnstashTargetMode.NewRemoteData.value
                 computer = self.inputs.metadata.get('computer')
                 with computer.get_transport() as transport:
-                    remote_user = transport.whoami_async()
+                    remote_user = transport.whoami()
                 remote_working_directory = computer.get_workdir().format(username=remote_user)
 
                 # The following line is set at calcjob::presubmit, but we need it here