flake.nix: Set system explicitly for now

Flakes now are evaluated in pure mode, so we can't rely on
currentSystem anymore.

flake.nix

@@ -6,7 +6,7 @@
   description = "A collection of packages for the Nix package manager";
 
   provides = flakes:
-    let pkgs = import ./. {}; in
+    let pkgs = import ./. { system = "x86_64-linux"; }; in
     {
       lib = import ./lib;