Make get_env_var return a reference to a Value (#13987)

# Description
Title says it all, changes `EngineState::get_env_var` to return a
`Option<&'a Value>` instead of an owned `Option<Value>`. This avoids
some unnecessary clones.

I also made a similar change to the `PluginExecutionContext` trait.

crates/nu-plugin-engine/src/context.rs

@@ -25,7 +25,7 @@ pub trait PluginExecutionContext: Send + Sync {
     /// Get plugin configuration
     fn get_plugin_config(&self) -> Result<Option<Value>, ShellError>;
     /// Get an environment variable from `$env`
-    fn get_env_var(&self, name: &str) -> Result<Option<Value>, ShellError>;
+    fn get_env_var(&self, name: &str) -> Result<Option<&Value>, ShellError>;
     /// Get all environment variables
     fn get_env_vars(&self) -> Result<HashMap<String, Value>, ShellError>;
     /// Get current working directory
@@ -125,7 +125,7 @@ impl<'a> PluginExecutionContext for PluginExecutionCommandContext<'a> {
             }))
     }
 
-    fn get_env_var(&self, name: &str) -> Result<Option<Value>, ShellError> {
+    fn get_env_var(&self, name: &str) -> Result<Option<&Value>, ShellError> {
         Ok(self.stack.get_env_var(&self.engine_state, name))
     }
 
@@ -300,7 +300,7 @@ impl PluginExecutionContext for PluginExecutionBogusContext {
         Ok(None)
     }
 
-    fn get_env_var(&self, _name: &str) -> Result<Option<Value>, ShellError> {
+    fn get_env_var(&self, _name: &str) -> Result<Option<&Value>, ShellError> {
         Err(ShellError::NushellFailed {
             msg: "get_env_var not implemented on bogus".into(),
         })