fix(nemesis): set_target_node should first give up current target

in PR #7016 there was a change that drop this part from the code
without it, we can get into case that nemesis which are calling this
function directly, might leave some nodes mark with `running_nemesis`
while no cleaup code can figure it out, and would unmark only the
current target selected.

Fixes: #7220

sdcm/nemesis.py

@@ -387,6 +387,9 @@ def set_target_node(self, dc_idx: Optional[int] = None, rack: Optional[int] = No
         if is_seed is DefaultValue - if self.filter_seed is True it act as if is_seed=False,
           otherwise it will act as if is_seed is None
         """
+        # first give up the current target node
+        self.unset_current_running_nemesis(self.target_node)
+
         with NEMESIS_TARGET_SELECTION_LOCK:
             nodes = self._get_target_nodes(is_seed=is_seed, dc_idx=dc_idx, rack=rack)
             if not nodes: