Revert "random: Do math as unsigned"

This reverts commit 0902e29f49.

Just doesn't work - overflows.

fish-rust/src/builtins/random.rs

@@ -165,7 +165,10 @@ pub fn random(
     };
 
     // Safe because end was a valid i64 and the result here is in the range start..=end.
-    let result: i64 = (start as u64 + (rand * step) as u64) as i64;
+    let result: i64 = start
+        .checked_add_unsigned(rand * step)
+        .and_then(|x| x.try_into().ok())
+        .unwrap();
 
     streams.out.append(sprintf!(L!("%d\n"), result));
     return STATUS_CMD_OK;