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-command/src/filesystem/cd.rs

@@ -107,7 +107,7 @@ impl Command for Cd {
         // Set OLDPWD.
         // We're using `Stack::get_env_var()` instead of `EngineState::cwd()` to avoid a conversion roundtrip.
         if let Some(oldpwd) = stack.get_env_var(engine_state, "PWD") {
-            stack.add_env_var("OLDPWD".into(), oldpwd)
+            stack.add_env_var("OLDPWD".into(), oldpwd.clone())
         }
 
         match have_permission(&path) {

crates/nu-command/src/network/http/client.rs

@@ -314,7 +314,7 @@ fn send_form_request(
         Value::List { ref vals, .. } => {
             if vals.len() % 2 != 0 {
                 return Err(ShellErrorOrRequestError::ShellError(ShellError::IncorrectValue {
-                    msg: "Body type 'list' for form requests requires paired values. E.g.: [foo, 10]".into(), 
+                    msg: "Body type 'list' for form requests requires paired values. E.g.: [foo, 10]".into(),
                     val_span: body.span(),
                     call_span: span,
                 }));
@@ -901,6 +901,7 @@ fn retrieve_http_proxy_from_env(engine_state: &EngineState, stack: &mut Stack) -
         .or(stack.get_env_var(engine_state, "https_proxy"))
         .or(stack.get_env_var(engine_state, "HTTPS_PROXY"))
         .or(stack.get_env_var(engine_state, "ALL_PROXY"))
+        .cloned()
         .and_then(|proxy| proxy.coerce_into_string().ok())
 }
 

crates/nu-command/src/viewers/griddle.rs

@@ -69,7 +69,7 @@ prints out the list properly."#
         let icons_param: bool = call.has_flag(engine_state, stack, "icons")?;
         let config = &stack.get_config(engine_state);
         let env_str = match stack.get_env_var(engine_state, "LS_COLORS") {
-            Some(v) => Some(env_to_string("LS_COLORS", &v, engine_state, stack)?),
+            Some(v) => Some(env_to_string("LS_COLORS", v, engine_state, stack)?),
             None => None,
         };
 

crates/nu-command/src/viewers/table.rs

@@ -623,7 +623,7 @@ fn handle_row_stream(
             let ls_colors_env_str = match input.stack.get_env_var(input.engine_state, "LS_COLORS") {
                 Some(v) => Some(env_to_string(
                     "LS_COLORS",
-                    &v,
+                    v,
                     input.engine_state,
                     input.stack,
                 )?),