treewide: revert switch to nixfmt

[GaetanLepage]

This PR aims at reverting the changes introduced in #1491.
It is my fault for not having reviewing it with more care.

The reasons for this proposition are the following:

• The nix fmt command now fails with openTempFileWithDefaultPermissions: permission denied (Read-only file system).
  Although the formatting actually goes through, this prevents someone from knowing whether the formatting "succeeded" or failed. I rely extensively on this in my programming workflow. This is being tracked in Remove recursive mode NixOS/nixfmt#151.
• This change implies manually rebasing + reformatting every single existing/future PR to account for the now failing git-hook and conflicts. I have a big load of such WIP branches.

Those reasons might feel very personal and sound egoist, but at the same time, I am the one writing the most code in this project and feel like my opinion should matter a little bit.
If some practical solutions to those issues exist (although I haven't find some yet), I would be glad to be proven wrong and close this.

I would then plead for this change to be temporarily reverted.
I want to stress that I am not against switching formatter in the long term. I actually do not care that much for the specific style we use. Although I like a lot the alejandra style, I understand the motivation to eventually mimic the future adoption of nixfmt by nixpkgs.

I also admit that I should have reviewed/tested this more carefully before approving it.