joypixels: Narrow unfree recommendation

JoyPixels requires explicit consent to the license and since the license
is unfree NixPkgs has to be configured to allow the package. This
updates the recommendation from setting allowUnfreePackages to adding
just JoyPixels to the allowUnfreePredicate.

pkgs/data/fonts/joypixels/default.nix

@@ -43,11 +43,16 @@ let
     unfree licenses.
 
     configuration.nix:
-      nixpkgs.config.allowUnfree = true;
+      nixpkgs.config.allowUnfreePredicate = pkg:
+        builtins.elem (lib.getName pkg) [
+          "joypixels"
+        ];
       nixpkgs.config.joypixels.acceptLicense = true;
 
     config.nix:
-      allowUnfree = true;
+      allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
+        "joypixels"
+      ];
       joypixels.acceptLicense = true;
 
     [1]: ${joypixels-free-license.url}