refactor(modules/plymouth): remove `substituteInPlace`

[trueNAHO]

To properly detect breaking changes, modules/plymouth/nixos.nix should not rely on substituteInPlace: https://discourse.nixos.org/t/when-to-use-substituteinplace-functions-vs-a-patch/11073.

For reference, here are the relevant sections:

https://github.com/danth/stylix/blob/a9e3ce064a778b386fb88fb152c02ae95aa2cbd2/modules/plymouth/nixos.nix#L32-L34 https://github.com/danth/stylix/blob/a9e3ce064a778b386fb88fb152c02ae95aa2cbd2/modules/plymouth/nixos.nix#L37-L39 https://github.com/danth/stylix/blob/a9e3ce064a778b386fb88fb152c02ae95aa2cbd2/modules/plymouth/nixos.nix#L66-L68