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.

tests/shell/pipeline/commands/internal.rs

@@ -392,7 +392,7 @@ fn let_env_hides_variable() {
     );
 
     assert_eq!(actual.out, "hello world");
-    assert!(actual.err.contains("cannot find column"));
+    assert!(actual.err.contains("not_found"));
 }
 
 #[test]
@@ -411,7 +411,7 @@ fn let_env_hides_variable_in_parent_scope() {
     );
 
     assert_eq!(actual.out, "hello world");
-    assert!(actual.err.contains("cannot find column"));
+    assert!(actual.err.contains("not_found"));
 }
 
 #[test]
@@ -424,7 +424,7 @@ fn unlet_env_variable() {
             echo $env.TEST_VAR
         "#
     );
-    assert!(actual.err.contains("cannot find column"));
+    assert!(actual.err.contains("not_found"));
 }
 
 #[test]
@@ -469,7 +469,7 @@ fn let_env_doesnt_leak() {
         "#
     );
 
-    assert!(actual.err.contains("cannot find column"));
+    assert!(actual.err.contains("not_found"));
 }
 
 #[test]
@@ -518,7 +518,7 @@ fn load_env_doesnt_leak() {
         "#
     );
 
-    assert!(actual.err.contains("cannot find column"));
+    assert!(actual.err.contains("not_found"));
 }
 
 #[test]