maint: Fix nix-expr tests by unsetting build hook

(cherry picked from commit 52b6cd6b7867dafe0906184e540c5195ba776ae9)

rust/nix-expr/src/eval_state.rs

@@ -532,7 +532,7 @@ pub fn test_init() {
     // would cause the test suite to reinvokes itself, causing an infinite loop.
     // While _NIX_TEST_NO_SANDBOX=1 should prevent this, we may also set the
     // build hook to "" to prevent this.
-    // settings::set("build-hook", "")?;
+    nix_util::settings::set("build-hook", "").unwrap();
 
     // When testing in the sandbox, the default build dir would be a parent of the storeDir,
     // which causes an error. So we set a custom build dir here.