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

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

These checks were done:

- built on NixOS
- ran ‘/nix/store/xp3sq5n9xcqwibdcwlnw1kndc67ykpb9-mlt-6.8.0/bin/melt -h’ got 0 exit code
- ran ‘/nix/store/xp3sq5n9xcqwibdcwlnw1kndc67ykpb9-mlt-6.8.0/bin/melt --help’ got 0 exit code
- ran ‘/nix/store/xp3sq5n9xcqwibdcwlnw1kndc67ykpb9-mlt-6.8.0/bin/melt help’ got 0 exit code
- ran ‘/nix/store/xp3sq5n9xcqwibdcwlnw1kndc67ykpb9-mlt-6.8.0/bin/.melt-wrapped -h’ got 0 exit code
- ran ‘/nix/store/xp3sq5n9xcqwibdcwlnw1kndc67ykpb9-mlt-6.8.0/bin/.melt-wrapped --help’ got 0 exit code
- ran ‘/nix/store/xp3sq5n9xcqwibdcwlnw1kndc67ykpb9-mlt-6.8.0/bin/.melt-wrapped help’ got 0 exit code
- found 6.8.0 with grep in /nix/store/xp3sq5n9xcqwibdcwlnw1kndc67ykpb9-mlt-6.8.0
- directory tree listing: https://gist.github.com/486acd53034e92704cc044a92c6e32ef
- du listing: https://gist.github.com/ff98100a51f78de609043361454df0b1

libsForQt5.mlt: 6.6.0 -> 6.8.0