colorschemes/github-theme: set colorscheme

[Prince213]

The default value of colorscheme is github-theme which isn't supported and breaks default configurations.

Errors look like:

Error in ~/.config/nvim/init.lua:                                                                                                            
E5113: Lua chunk: vim/_editor.lua:0: ~/.config/nvim/init.lua..nvim_exec2() called at ~/.config/nvim/init.lua:0, line 1: Vim(col
orscheme):E185: Cannot find color scheme 'github-theme'                                                                                                    
stack traceback:                                                                                                                                           
        [C]: in function 'nvim_exec2'                                                                                                                      
        vim/_editor.lua: in function 'cmd'                                                                                                                 
        ~/.config/nvim/init.lua:7: in main chunk

[nixvim-ci]

Successfully created backport PR for nixos-25.11:

• [Backport nixos-25.11] colorschemes/github-theme: set colorscheme #4110