refactor(tvix/eval): use &Path instead of PathBuf

This allows getting rid of some clones in eval/src/vm/generators.rs.

Change-Id: I330390307d3bcfeef19c98954c753ee55b1ccee3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8604
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/nix_search_path.rs

@@ -85,7 +85,7 @@ impl NixSearchPathEntry {
             }
         };
 
-        if io.path_exists(path.clone()).map_err(|e| ErrorKind::IO {
+        if io.path_exists(&path).map_err(|e| ErrorKind::IO {
             path: Some(path.clone()),
             error: e.into(),
         })? {