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.

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

@@ -194,7 +194,7 @@ fn source_env_eval_export_env_hide() {
 
         let actual = nu!(cwd: dirs.test(), pipeline(&inp.join("; ")));
 
-        assert!(actual.err.contains("cannot find column"));
+        assert!(actual.err.contains("not_found"));
     })
 }
 

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

@@ -97,7 +97,7 @@ fn use_eval_export_env_hide() {
 
         let actual = nu!(cwd: dirs.test(), pipeline(&inp.join("; ")));
 
-        assert!(actual.err.contains("cannot find column"));
+        assert!(actual.err.contains("not_found"));
     })
 }