nixfmt-rfc-style migrate to nixfmt & keep nixfmt-classic.

**Is your feature request related to a problem? Please describe.** <!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] --> Pr: https://github.com/NixOS/nixpkgs/pull/425068 has forced the migration to nixfmt and nixfmt-rfc-style will be removed in future. ``` evaluation warning: nixfmt-rfc-style is now the default for the 'nixfmt' formatter. 'nixfmt-rfc-style' is deprecated and will be removed in the future. ``` <!-- A clear and concise description of what you want to happen. --> We need to migrate to nixfmt, remove nixfmt-rfc-style & keep nixfmt-classic. <!-- A clear and concise description of any alternative solutions or features you've considered. --> There is no alternatives that I could think. **Additional context** CC @NixOS/CI I am considering to make a pr from DigitalBrewStudios fork. <!-- Add any other context or screenshots about the feature request here. -->