lock flake-compat to root's input

[ElvishJerricco]

This isn't often relevant. But if you have inputs that themselves have a flake-compat input, you can end up using those by mistake. This locks the version used by default.nix to the input of the root node instead of which ever input happens to take the flake-compat name first.

For example, I had my flake set up with #4, and eventually my default.nix stopped working because one of my inputs got updated and its flake-compat input ended up taking the flake-compat name in flake.lock, while mine was under flake-compat_2.

[zimbatm]

thanks!