fix: use environment variables to prevent command_not_found from recursing (#11090)

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- fixes #11014

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When the `command_not_found` hook is entered, we set an environment
variable for context. If an unknown command is encountered and the
`command_not_found` context environment variable is already present, it
implies a command in the hook closure is also not found. We stop the
recursion right there.

# User-Facing Changes
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Incorrect `command_not_found` hooks can be caught without panicking.

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Tests are passing.

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crates/nu-command/src/system/run_external.rs

@@ -401,25 +401,41 @@ impl ExternalCommand {
                             let mut engine_state = engine_state.clone();
                             if let Some(hook) = engine_state.config.hooks.command_not_found.clone()
                             {
+                                let canary = "ENTERED_COMMAND_NOT_FOUND";
                                 let stack = &mut stack.start_capture();
-                                if let Ok(PipelineData::Value(Value::String { val, .. }, ..)) =
-                                    eval_hook(
+                                if stack.has_env_var(&engine_state, canary) {
+                                    return Err(ShellError::ExternalCommand {
+                                        label: "command_not_found handler could not be run".into(),
+                                        help: "make sure the command_not_found closure itself does not use unknown commands".to_string(),
+                                        span: self.name.span,
+                                    });
+                                }
+                                stack.add_env_var(
+                                    canary.to_string(),
+                                    Value::bool(true, Span::unknown()),
+                                );
+                                match eval_hook(
                                     &mut engine_state,
                                     stack,
                                     None,
                                     vec![(
                                         "cmd_name".into(),
-                                            Value::string(
-                                                self.name.item.to_string(),
-                                                self.name.span,
-                                            ),
+                                        Value::string(self.name.item.to_string(), self.name.span),
                                     )],
                                     &hook,
                                     "command_not_found",
-                                    )
-                                {
+                                ) {
+                                    Ok(PipelineData::Value(Value::String { val, .. }, ..)) => {
                                         err_str = format!("{}\n{}", err_str, val);
                                     }
+
+                                    Err(err) => {
+                                        stack.remove_env_var(&engine_state, canary);
+                                        return Err(err);
+                                    }
+                                    _ => {}
+                                }
+                                stack.remove_env_var(&engine_state, canary);
                             }
                         }