fix(tvix/eval): allow reading non-UTF8 files

With our values using bstr now, we're not restricted to only reading
files that contain valid UTF-8.

Update our `read_to_string` function to `read_to_end`
(named like `std::io::Read::read_to_end`), and have it return a Vec<u8>.

Change-Id: I87f0291dc855a132689576559c891d66c30ddf2b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11003
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: Pádraic Ó Mhuiris <patrick.morris.310@gmail.com>
Reviewed-by: flokli <flokli@flokli.de>

tvix/eval/src/vm/generators.rs

@@ -429,7 +429,7 @@ where
                             let content = self
                                 .io_handle
                                 .as_ref()
-                                .read_to_string(&path)
+                                .read_to_end(&path)
                                 .map_err(|e| ErrorKind::IO {
                                     path: Some(path),
                                     error: e.into(),