Avoid the infinite recursion

The new binding completely obscures the old binding.  The `else` case was
recursive with no base.

default.nix

@@ -8,9 +8,10 @@
 let
   pkgs' = pkgs.extend (import ./overlays.nix);
   callPackage = pkgs'.python27Packages.callPackage;
-  tahoe-lafs = if tahoe-lafs == null then pkgs.python2Packages.tahoe-lafs-1_14 else tahoe-lafs;
+  tahoe-lafs' = if tahoe-lafs == null then pkgs.python2Packages.tahoe-lafs-1_14 else tahoe-lafs;
 in
 callPackage ./zkapauthorizer.nix {
   challenge-bypass-ristretto = callPackage ./python-challenge-bypass-ristretto.nix { };
-  inherit hypothesisProfile collectCoverage testSuite trialArgs tahoe-lafs;
+  inherit hypothesisProfile collectCoverage testSuite trialArgs;
+  tahoe-lafs = tahoe-lafs';
 }