chore(tvix/eval): gate tvix_eval::StdIO behind the impure feature

This shouldn't be available if we've built a "pure" crate.

Change-Id: I7c85827ee212890252ff7e0b6242e2c52618cba5
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7572
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tvix/eval/src/io.rs

@@ -27,8 +27,10 @@ pub trait EvalIO {
 
 /// Implementation of [`EvalIO`] that simply uses the equivalent
 /// standard library functions, i.e. does local file-IO.
+#[cfg(feature = "impure")]
 pub struct StdIO;
 
+#[cfg(feature = "impure")]
 impl EvalIO for StdIO {
     fn read_to_string(&self, path: PathBuf) -> Result<String, ErrorKind> {
         let path: PathBuf = path.into();