feat(tvix/eval): Allow adding strings to paths

Implement adding paths and strings via OpAdd. Since the nix rules are
quite obscure, I'm electing to test this one with an oracle test to
avoid the danger of getting the actual asserted result wrong.

Change-Id: Icdcca3690ca2e8459e386c1f29cc48eaaa39e9a3
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6914
Autosubmit: grfn <grfn@gws.fyi>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tvix/eval/src/value/mod.rs

@@ -383,6 +383,12 @@ impl From<String> for Value {
     }
 }
 
+impl From<PathBuf> for Value {
+    fn from(path: PathBuf) -> Self {
+        Self::Path(path)
+    }
+}
+
 fn type_error(expected: &'static str, actual: &Value) -> ErrorKind {
     ErrorKind::TypeError {
         expected,