flake: disable allow-import-from-derivation

Unfortunately this will print a confirmation when first using the flake:

```
do you want to allow configuration setting 'allow-import-from-derivation' to be set to 'false' (y/N)?
```

This is no different to the cachix configs though, so probably no big
deal.

flake.nix

@@ -71,6 +71,7 @@
     extra-trusted-public-keys = [
       "nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs="
     ];
+    allow-import-from-derivation = false;
   };
 
   outputs =