src/timeout: fix datetime error

Fix the below error:
```
File "/home/kernelci/pipeline/src/timeout.py", line 119, in _get_available_nodes
    'holdoff__lt': datetime.isoformat(datetime.utcnow()),
                   ^^^^^^^^^^^^^^^^^^
AttributeError: module 'datetime' has no attribute 'isoformat'
```
Related to: #1277

Signed-off-by: Jeny Sadadia <[email protected]>

Author: Jeny Sadadia

src/timeout.py

@@ -116,7 +116,7 @@ def __init__(self, configs, args):
     def _get_available_nodes(self):
         nodes = self._api.node.find({
             'state': 'available',
-            'holdoff__lt': datetime.isoformat(datetime.utcnow()),
+            'holdoff__lt': datetime.datetime.isoformat(datetime.datetime.now(datetime.UTC)),
         })
         return {node['id']: node for node in nodes}