maint: Unset substituters in nix-expr tests

Runs offline, and would only slow things down.

(cherry picked from commit ea3ef1c59a1fe8e380302acc1885877a1ac8e0b6)

rust/nix-expr/src/eval_state.rs

@@ -538,6 +538,9 @@ pub fn test_init() {
     // which causes an error. So we set a custom build dir here.
     nix_util::settings::set("sandbox-build-dir", "/custom-build-dir-for-test").unwrap();
     std::env::set_var("_NIX_TEST_NO_SANDBOX", "1");
+
+    // The tests run offline
+    nix_util::settings::set("substituters", "").unwrap();
 }
 
 #[cfg(test)]