Fix unused import in test when built without std

warning: unused import: `std::path::PathBuf`
     --> tests/test_expr.rs:4:5
      |
    4 | use std::path::PathBuf;
      |     ^^^^^^^^^^^^^^^^^^
      |
      = note: `#[warn(unused_imports)]` on by default

tests/test_expr.rs

@@ -1,6 +1,7 @@
 #![allow(clippy::iter_cloned_collect, clippy::uninlined_format_args)]
 
 use core::fmt::Display;
+#[cfg(feature = "std")]
 use std::path::PathBuf;
 use thiserror::Error;