Agent: Use private module variable for ID caching

mypy complains about setting an attribute on the function object. Using
`global _id` accomplishes the same thing and doesn't upset mypy

monkey/infection_monkey/utils/ids.py

@@ -2,6 +2,8 @@ from uuid import UUID, getnode, uuid4
 
 from common.types import HardwareID
 
+_id = None
+
 
 def get_agent_id() -> UUID:
     """
@@ -10,13 +12,11 @@ def get_agent_id() -> UUID:
     Each time an agent process starts, the return value of this function will be unique. Subsequent
     calls to this function from within the same process will have the same return value.
     """
-    if get_agent_id._id is None:
-        get_agent_id._id = uuid4()
+    global _id
+    if _id is None:
+        _id = uuid4()
 
-    return get_agent_id._id
-
-
-get_agent_id._id = None
+    return _id
 
 
 def get_machine_id() -> HardwareID: