Make env-related tests more resilient (#7423)

# Description

Fixes https://github.com/nushell/nushell/issues/6708

The error message of environment variable not found could change
depending on the `$env` content which can produce random failures on
different systems. This PR hopefully makes the tests more resilient.

# User-Facing Changes

None

# 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.

src/tests/test_hiding.rs

@@ -116,7 +116,7 @@ fn hides_alias_in_scope_4() -> TestResult {
 fn hides_env_in_scope_1() -> TestResult {
     fail_test(
         r#"let-env foo = "foo"; do { hide-env foo; $env.foo }"#,
-        "cannot find column",
+        "not_found",
     )
 }