mlt: 6.6.0 -> 6.8.0 (#40653)

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
- /nix/store/xsmh09whsj39h5hc7g0ljj344i5i70zk-mlt-6.8.0/bin/melt passed the binary check.
- /nix/store/xsmh09whsj39h5hc7g0ljj344i5i70zk-mlt-6.8.0/bin/.melt-wrapped passed the binary check.
- 2 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 6.8.0 with grep in /nix/store/xsmh09whsj39h5hc7g0ljj344i5i70zk-mlt-6.8.0
- directory tree listing: https://gist.github.com/3a7f7c995e0d2d7232ab28093ddd4613
- du listing: https://gist.github.com/c8e940fb79b09b1aeee2243a1fff57ca