Use nixpkgs version from flake by default

When not using flakes, you would immediately start using the latest
version of the nix-ros branch whenever I updated it, even if it hadn't
been tested or added to the cache yet. Now, it will match the flake
locked version by default even when not using flakes.

default.nix

@@ -1,4 +1,11 @@
-{ nixpkgs ? builtins.fetchTarball "https://github.com/lopsided98/nixpkgs/archive/nix-ros.tar.gz"
+let
+  lock = builtins.fromJSON (builtins.readFile ./flake.lock);
+  lockedNixpkgs = builtins.fetchTarball {
+    url = "https://github.com/lopsided98/nixpkgs/archive/${lock.nodes.nixpkgs.locked.rev}.tar.gz";
+    sha256 = lock.nodes.nixpkgs.locked.narHash;
+  };
+in
+{ nixpkgs ? lockedNixpkgs
 , overlays ? []
 , ... }@args: