nix_eval_jobs: Remove --impure

Note that this still does not make flake evaluation pure in
nix-eval-jobs yet, since it forcibly resets `evalSettings.pureEval`
prior to starting the evaluation.

src/nix/evaluator/nix_eval_jobs.rs

@@ -86,7 +86,6 @@ impl DrvSetEvaluator for NixEvalJobs {
 
         let mut command = Command::new(&self.executable);
         command
-            .arg("--impure")
             .arg("--workers")
             .arg(self.workers.to_string())
             .arg(&expr_file);