lib: explicitly write down () when collecting to Result<()>

Apparently inference ran afoul of rust-lang/rust#123748

src/lib.rs

@@ -574,7 +574,7 @@ pub fn copy_all_spool_paths(
     to_copy
         .into_par_iter()
         .map(|path| copy_spool_path(&path))
-        .collect::<Result<_>>()?;
+        .collect::<Result<()>>()?;
     Ok(())
 }