fix 'let' to properly redirect (#10360)

# Description

Fixes a bug in `let` where the pipeline wasn't being properly
redirected.

fixes #9767

# User-Facing Changes

Shouldn't have any breaking changes, as this should be better for
expected use cases.

# Tests + Formatting
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crates/nu-cmd-lang/src/core_commands/let_.rs

@@ -61,14 +61,7 @@ impl Command for Let {
             .expect("internal error: missing right hand side");
 
         let block = engine_state.get_block(block_id);
-        let pipeline_data = eval_block(
-            engine_state,
-            stack,
-            block,
-            input,
-            call.redirect_stdout,
-            call.redirect_stderr,
-        )?;
+        let pipeline_data = eval_block(engine_state, stack, block, input, true, false)?;
         stack.add_var(var_id, pipeline_data.into_value(call.head));
         Ok(PipelineData::empty())
     }

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

@@ -46,6 +46,20 @@ fn mut_pipeline_allows_in() {
     assert_eq!(actual.out, "21");
 }
 
+#[test]
+fn let_pipeline_redirects_internals() {
+    let actual = nu!(r#"let x = echo 'bar'; $x | str length"#);
+
+    assert_eq!(actual.out, "3");
+}
+
+#[test]
+fn let_pipeline_redirects_externals() {
+    let actual = nu!(r#"let x = nu --testbin cococo 'bar'; $x | str length"#);
+
+    assert_eq!(actual.out, "3");
+}
+
 #[ignore]
 #[test]
 fn let_with_external_failed() {