Avoid unnecessary warning message when plotting is not enabled (#2380)

1. Please describe how your code solves the related issue.
import graphviz seems also introduces a set of warning message like 
```
DEBUG    graphviz._tools:_tools.py:147 deprecate positional args: graphviz.graphs.BaseGraph.__init__(['comment', 'filename', 'directory', 'format', 'engine', 'encoding', 'graph_attr', 'node_attr', 'edge_attr', 'body', 'strict'])
DEBUG    graphviz._tools:_tools.py:147 deprecate positional args: graphviz.sources.Source.from_file(['directory', 'format', 'engine', 'encoding', 'renderer', 'formatter'])
DEBUG    graphviz._tools:_tools.py:147 deprecate positional args: graphviz.sources.Source.__init__(['filename', 'directory', 'format', 'engine', 'encoding'])
DEBUG    graphviz._tools:_tools.py:147 deprecate positional args: graphviz.sources.Source.save(['directory'])
```
the warnings are very noisy even if plotting is disabled. move the
import below the check of plotting enablement so that those debugging
message does not shown up if plotting is disabled.

src/snowflake/snowpark/_internal/compiler/utils.py

@@ -357,14 +357,14 @@ def plot_plan_if_enabled(root: LogicalPlan, filename: str) -> None:
     """
     import os
 
-    import graphviz  # pyright: ignore[reportMissingImports]
-
     if (
         os.environ.get("ENABLE_SNOWPARK_LOGICAL_PLAN_PLOTTING", "false").lower()
         != "true"
     ):
         return
 
+    import graphviz  # pyright: ignore[reportMissingImports]
+
     def get_stat(node: LogicalPlan):
         def get_name(node: Optional[LogicalPlan]) -> str:
             if node is None: