Use nixpkgs from flake.lock and support attrs when importing bscpkgs

This makes `nix-build` and friends use the current flake lock instead of
the outdated pinned version we had in `./nixpkgs.nix`

With this, `nix-build -A ovni` and `nix build .#ovni` will produce the
same result.

This will fail if the flake nixpkgs input does not come from NixOS/nixpkgs.
We could use edolstra/flake-compat instead, but it's overkill imho.

Additionally, I made default.nix behave like nixpkgs, so that we can
import bscpkgs à la nixpkgs (Apply overlays and other options that nixpkgs
accepts): `import bscpkgs { overlays = [ <...> ]; }`

default.nix

@@ -1,11 +1,19 @@
 let
   bscOverlay = import ./overlay.nix;
 
-  # Pin the nixpkgs
-  nixpkgsPath = import ./nixpkgs.nix;
-
-  pkgs = import nixpkgsPath {
-    overlays = [ bscOverlay ];
+  # read flake.lock and determine revision from there
+  lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+  inherit (lock.nodes.nixpkgs.locked) rev narHash;
+  fetchedNixpkgs = builtins.fetchTarball {
+    url = "https://github.com/NixOS/nixpkgs/archive/${rev}.tar.gz";
+    sha256 = narHash;
   };
-
-in pkgs
+in
+{ overlays ? [ ]
+, nixpkgs ? fetchedNixpkgs
+, ...
+}@attrs:
+import nixpkgs (
+  (builtins.removeAttrs attrs [ "overlays" "nixpkgs" ]) //
+  { overlays = [ bscOverlay ] ++ overlays; }
+)

flake.nix

@@ -13,5 +13,9 @@
       bscOverlay = import ./overlay.nix;
       overlays.default = self.bscOverlay;
       legacyPackages.x86_64-linux = pkgs;
+
+      hydraJobs = {
+        inherit (pkgs) bsc-ci;
+      };
     };
 }

nixpkgs.nix

@@ -1,9 +0,0 @@
-let
-  commit = "e4ad989506ec7d71f7302cc3067abd82730a4beb";
-in builtins.fetchTarball {
-    # Descriptive name to make the store path easier to identify
-    name = "nixpkgs-${commit}";
-    url = "https://github.com/nixos/nixpkgs/archive/${commit}.tar.gz";
-    # Hash obtained using `nix-prefetch-url --unpack <url>`
-    sha256 = "sha256-de9KYi8rSJpqvBfNwscWdalIJXPo8NjdIZcEJum1mH0=";
-}