unetbootin: 657 -> 661

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/unetbootin/versions.

These checks were done:

- built on NixOS
- Warning: no invocation of /nix/store/25pjsiav4kpzyvms0a24zfmm3267nsq9-unetbootin-661/bin/unetbootin had a zero exit code or showed the expected version
- Warning: no invocation of /nix/store/25pjsiav4kpzyvms0a24zfmm3267nsq9-unetbootin-661/bin/.unetbootin-wrapped had a zero exit code or showed the expected version
- 0 of 2 passed binary check by having a zero exit code.
- 0 of 2 passed binary check by having the new version present in output.
- found 661 with grep in /nix/store/25pjsiav4kpzyvms0a24zfmm3267nsq9-unetbootin-661
- directory tree listing: https://gist.github.com/b36c6d9dbe30c7b0e794715dafed8272
- du listing: https://gist.github.com/2da554212740e7e9655de8e211d2e8ed