fix(tvix/eval): NixString are bytes

This is not necessarily valid UTF-8.

Change-Id: I72f3157240772eb9c558e5699b4785e44d256fd4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11702
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -1505,7 +1505,7 @@ mod pure_builtins {
 
         let mut buf: Vec<u8> = vec![];
         to_xml::value_to_xml(&mut buf, &value)?;
-        Ok(String::from_utf8(buf)?.into())
+        Ok(buf.into())
     }
 
     #[builtin("placeholder")]