Update to latest nixpkgs unstable.

This switches from my nixpkgs fork back to upstream nixpkgs,
which thanks to community-sponsored CI has been shown to be
green for a while.

Not carrying my changes to `pkgsStatic` breaks support for
using that as a base, but that was only an experiment on
my side anyway. `pkgsMusl` is the better base.

Author: nh2

nixpkgs

@@ -17,4 +17,4 @@ if builtins.getEnv "STATIC_HASKELL_NIX_CI_NIXPKGS_UNSTABLE_BUILD" == "1"
     if builtins.pathExists ./nixpkgs/pkgs
       then import ./nixpkgs {}
       # Pinned nixpkgs version; should be kept up-to-date with our submodule.
-      else import (fetchTarball https://github.com/nh2/nixpkgs/archive/6b43b1d11bb67607919256ea5b8a29aec770f1f7.tar.gz) {}
+      else import (fetchTarball https://github.com/NixOS/nixpkgs/archive/0c960262d159d3a884dadc3d4e4b131557dad116.tar.gz) {}

nixpkgs.nix

@@ -21,6 +21,9 @@ in
   overlays ? [],
 
   approach ? # "pkgsMusl" or "pkgsStatic"
+    # TODO `pkgsStatic` support is currently not maintained and will likely be removed,
+    #      because `pkgsMusl` is a better base for what we need.
+    #      See https://github.com/NixOS/nixpkgs/issues/61575
     # TODO Find out why `pkgsStatic` creates ~3x larger binaries.
     "pkgsMusl", # does not exercise cross compilation
     # "pkgsStatic", # exercises cross compilation