fix(tvix/eval/nix_search_path): gate tests on impure feature

These use StdIO, which is only available if the impure feature is
enabled.

Change-Id: I18d8e191a7eba6ba5bd59f43631973eaa796c7bb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11721
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/nix_search_path.rs

@@ -197,6 +197,8 @@ mod tests {
         }
     }
 
+    // this uses StdIO, which is only available with the impure feature.
+    #[cfg(feature = "impure")]
     mod resolve {
         use crate::StdIO;
         use path_clean::PathClean;