github actions: niv -> flakes

Use flake.nix/lock as a single source of truth for resolving
dependencies

NOTE: this patch implies a dependency update as well.

default.nix

@@ -66,10 +66,10 @@
 , writeTextDir }:
 
 let
-  sources = import ./nix/sources.nix;
+  flake = import ./flake-compat-helper.nix { src=./.; };
   lock = p: if dependencyOverrides ? ${p}
             then dependencyOverrides.${p}
-            else sources.${p};
+            else flake.inputs.${p};
   # Packages we need to get the default doom configuration run
   overrides = self: super:
     (pkgs.callPackage ./overrides.nix { inherit lock; } self super) // (emacsPackagesOverlay self super);