cat: remove hardcoded var from bench

Co-authored-by: xtqqczze <45661989+xtqqczze@users.noreply.github.com>

Author: oech3

src/uu/cat/benches/cat_bench.rs

@@ -2,8 +2,8 @@ use divan::{Bencher, black_box};
 use uu_cat::uumain;
 use uucore::benchmark::{run_util_function, setup_test_file};
 
-#[divan::bench]
-fn bench_cat_15kib(bencher: Bencher) {
+#[divan::bench(args = [10_000])]
+fn cat_default(bencher: Bencher, size_bytes: usize) {
     let size = 15 * 1024;
     let data = vec![b'a'; size];