fix(tvix/eval): allow negative substring lengths

Nix uses string::substr without checking the sign of the length[1].
The NixOS testing infrastructure relies on this[2], and on the
implicit conversion of that to the maximum possible value for a
size_t.

[1]: ecae62020b/src/libexpr/primops.cc (L3597)
[2]: c7c2984716/nixos/lib/testing/driver.nix (L29)

Change-Id: I6d0caf6830b6bda3fdf44c40c81de6a1befeca7b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8746
Reviewed-by: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI

tvix/eval/src/builtins/mod.rs

@@ -869,12 +869,11 @@ mod pure_builtins {
             return Ok(Value::String("".into()));
         }
 
-        if len < 0 {
-            return Err(ErrorKind::NegativeLength { length: len });
-        }
-
-        let len = len as usize;
-        let end = cmp::min(beg + len, x.as_str().len());
+        let end = if len < 0 {
+            x.as_str().len() as usize
+        } else {
+            cmp::min(beg + (len as usize), x.as_str().len())
+        };
 
         Ok(Value::String(x.as_str()[beg..end].into()))
     }