added deprecated note (#1584)

Replaced the informal deprecation notice in the doc comment of
`CustomNode::envs` with a proper `Rust #[deprecated]` attribute, so the
compiler emits warnings when this field is used.

Author: haixuanTao

libraries/message/src/descriptor.rs

@@ -678,9 +678,8 @@ pub struct CustomNode {
     /// Args for the executable.
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub args: Option<String>,
-    /// Environment variables for the custom nodes
-    ///
-    /// Deprecated, use outer-level `env` field instead.
+    /// Environment variables for the custom nodes.
+    #[deprecated(note = "Use the outer-level `env` field on `Node` instead")]
     pub envs: Option<BTreeMap<String, EnvValue>>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub build: Option<String>,