Support building nextpnr with nix flakes

[RCoeurjoly]

So far GUI is disabled, because nixpkgs has only qt6, and not qt5.

If enabling the GUI is required, with a little more love it should be doable.

[rowanG077]

I would like this see the flake replace the shell.nix. Important there are the environment variables that need to be set so cmake can find the deps in the dev shell.

[RCoeurjoly]

I would like this see the flake replace the shell.nix. Important there are the environment variables that need to be set so cmake can find the deps in the dev shell.

I have added a devShell to flake.nix, and the shellHook to it, copied from shell.nix (thanks for the pointer!).

However, when inside the devShell, I am unable to compile nextpnr.
I have tried several architectures (all, ice40, ecp5).

All fail.

Maybe leave the devShell as it is, a WIP?