Make exit code available in catch block (#12648)

# Description
Bandaid fix for #12643, where it is not possible to get the exit code of
a failed external command while also having the external command inherit
nushell's stdout and stderr. This changes `try` so that the exit code of
external command is available in the `catch` block via the usual
`$env.LAST_EXIT_CODE`.

# Tests + Formatting
Added one test.

# After Submitting
Rework I/O redirection and possibly exit codes.

crates/nu-cmd-lang/src/core_commands/try_.rs

@@ -64,9 +64,8 @@ impl Command for Try {
             Ok(pipeline) => {
                 let (pipeline, external_failed) = pipeline.check_external_failed();
                 if external_failed {
-                    // Because external command errors aren't "real" errors,
+                    let exit_code = pipeline.drain_with_exit_code()?;
-                    // (unless do -c is in effect)
+                    stack.add_env_var("LAST_EXIT_CODE".into(), Value::int(exit_code, call.head));
-                    // they can't be passed in as Nushell values.
                     let err_value = Value::nothing(call.head);
                     handle_catch(err_value, catch_block, engine_state, stack, eval_block)
                 } else {

crates/nu-command/tests/commands/try_.rs

@@ -93,3 +93,9 @@ fn can_catch_infinite_recursion() {
         "#);
     assert_eq!(actual.out, "Caught infinite recursion");
 }
+
+#[test]
+fn exit_code_available_in_catch() {
+    let actual = nu!("try { nu -c 'exit 42' } catch { $env.LAST_EXIT_CODE }");
+    assert_eq!(actual.out, "42");
+}