bug: doesn't work in pure evaluation mode

```
nix eval .#rng
error:
… while evaluating the attribute 'int'

at /nix/store/h48bkrwv159k9y8gqnz0h86i0mrvgdwr-source/default.nix:25:5:

24| rng = seed: fix (self: {
25| int = (fromTOML "x=0x${substring 28 8 (hash seed)}").x;
| ^
26|

… while calling the 'fromTOML' builtin

at /nix/store/h48bkrwv159k9y8gqnz0h86i0mrvgdwr-source/default.nix:25:12:

24| rng = seed: fix (self: {
25| int = (fromTOML "x=0x${substring 28 8 (hash seed)}").x;
| ^
26|

(stack trace truncated; use '--show-trace' to show the full trace)

error: access to absolute path '/proc/sys/kernel/random/uuid' is forbidden in pure eval mode (use '--impure' to override)
```