check external failed in let assignment (#8164)

# Description

Fixes: #8136

# User-Facing Changes

The following command

```
let VAR = ^cat non-existing-file; echo "failed"
```
will no longer output `failed` message


# Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

---------

Co-authored-by: Reilly Wood <reilly.wood@icloud.com>

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

@@ -25,3 +25,13 @@ fn let_doesnt_mutate() {
 
     assert!(actual.err.contains("immutable"));
 }
+
+#[test]
+fn let_with_external_failed() {
+    let actual = nu!(
+        cwd: ".",
+        pipeline(r#"let x = nu --testbin outcome_err "aa"; echo fail"#)
+    );
+
+    assert!(!actual.out.contains("fail"));
+}