Revert "Remove env stuff from input reading"

I remember now. This was so I can `cargo run bigboy_input` without code changes

2021/src/common.rs

@@ -1,5 +1,13 @@
+use std::env;
+
 pub fn read_file(day: usize) -> String {
-    std::fs::read_to_string(format!("inputs/day{:0>2}", day)).unwrap()
+    std::fs::read_to_string(
+        env::args()
+            .nth(1)
+            .filter(|n| n != "--bench")
+            .unwrap_or_else(|| format!("inputs/day{:0>2}", day)),
+    )
+    .unwrap()
 }
 
 #[inline]