refactor(tvix/eval): box PathBuf

This shaves another 8 bytes off Value. How did that type get so big?!

Change-Id: I65e9b59a1636bd57e3cc4aec5fea16887070b832
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8153
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Tested-by: BuildkiteCI

tvix/eval/src/value/arbitrary.rs

@@ -92,7 +92,7 @@ fn leaf_value() -> impl Strategy<Value = Value> {
         any::<i64>().prop_map(Integer),
         any::<f64>().prop_map(Float),
         any::<NixString>().prop_map(String),
-        any::<OsString>().prop_map(|s| Path(s.into())),
+        any::<OsString>().prop_map(|s| Path(Box::new(s.into()))),
     ]
 }