set nix.nixPath to allow imperatively installing packages

.forgejo/workflows/update.yaml

@@ -11,7 +11,7 @@ jobs:
     steps:
       - name: Checkout repository
         uses: actions/checkout@v4
-      - name: Install npins
+      - name: Update dependencies
         run: nix-shell --run "npins update"
       - name: Create PR
         uses: peter-evans/create-pull-request@v7

infra/common/nixos/default.nix

@@ -2,6 +2,7 @@
 
 let
   inherit (lib) mkDefault;
+  sources = import ../../../npins;
 
 in
 {
@@ -22,4 +23,6 @@ in
   nix.extraOptions = ''
     experimental-features = nix-command flakes
   '';
+
+  nix.nixPath = lib.mapAttrsToList (k: v: k + "=" + v) sources;
 }