Fix the Nix build

We use the Nix "jailbreak" function to ignore the upper bounds
on the CGI package dependency.

gf.nix

@@ -1,3 +1,19 @@
-let pkgs = import <nixpkgs> {}; in rec {
-  gf = pkgs.haskellPackages.callPackage ./default.nix {};
+let
+  # We import the package hierarchy from the NIX_PATH.
+  pkgs = import <nixpkgs> {};
+
+  #
+  # The `cgi` package has some out-of-date upper bounds.
+  # Instead of fixing that upstream, we can use the "jailbreak"
+  # function in Nix, which patches away the upper bounds.
+  #
+  # This isn't ideal, but it lets us build the dependency.
+  #
+  jailbreak = pkgs.haskell.lib.doJailbreak;
+  haskellPackages = pkgs.haskellPackages.extend (self: super: {
+    cgi = jailbreak super.cgi;
+  });
+
+in {
+  gf = haskellPackages.callPackage (import ./default.nix) {};
 }