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.

flake.nix

@@ -68,14 +68,30 @@
     revealjs.flake = false;
     rotate-text.url = "github:debug-ito/rotate-text.el";
     rotate-text.flake = false;
+
+    nixpkgs.url = "nixpkgs/nixpkgs-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
   };
 
-  outputs = { nixpkgs, ... }@inputs: {
-    hmModule = import ./modules/home-manager.nix inputs;
-    checks."x86_64-linux".init-example-el =
-      let
-        pkgs = nixpkgs.legacyPackages."x86_64-linux";
-      in
-        pkgs.callPackage ./. { doomPrivateDir = ./test/doom.d; dependencyOverrides = inputs; };
-  };
+  outputs = { nixpkgs, flake-utils, ... }@inputs:
+    let
+      inherit (flake-utils.lib) eachDefaultSystem eachSystem;
+    in
+    eachDefaultSystem
+      (system:
+        let
+          pkgs = import nixpkgs { inherit system; };
+        in
+        {
+          devShell = pkgs.mkShell { buildInputs = [ (pkgs.python3.withPackages (ps: with ps; [ PyGithub ])) ]; };
+        }) //
+    eachSystem [ "x86_64-linux" ]
+      (system: {
+        checks = {
+          init-example-el = nixpkgs.legacyPackages.${system}.callPackage ./. { doomPrivateDir = ./test/doom.d; dependencyOverrides = inputs; };
+        };
+      }) //
+    {
+      hmModule = import ./modules/home-manager.nix inputs;
+    };
 }