chore: simplify how blns is loaded into tests (#4705)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

Similarly to #4703, this PR simplifies how we load in the blns file for
tests.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.

Author: TomAFrench

compiler/noirc_frontend/build.rs

@@ -1160,7 +1160,7 @@ mod tests {
     fn test_big_list_of_naughty_strings() {
         use std::mem::discriminant;
 
-        let blns_contents = include_str!(env!("BLNS_JSON_PATH"));
+        let blns_contents = include_str!("./blns/blns.base64.json");
         let blns_base64: Vec<String> =
             serde_json::from_str(blns_contents).expect("BLNS json invalid");
         for blns_base64_str in blns_base64 {