joypixels: Move assert to allow override

By moving the assert concerning license acceptance into the src
attribute license acceptance can be expressed with an override,
`joypixels.override { acceptLicense = true; }`.

pkgs/data/fonts/joypixels/default.nix

@@ -34,9 +34,7 @@ let inherit (stdenv.hostPlatform.parsed) kernel;
       free = false;
     };
 
-in
+    throwLicense = throw ''
-
-assert !acceptLicense -> throw ''
       Use of the JoyPixels font requires acceptance of the license.
         - ${joypixels-free-license.fullName} [1]
         - ${joypixels-license-appendix.fullName} [2]
@@ -55,13 +53,16 @@ assert !acceptLicense -> throw ''
 
       [1]: ${joypixels-free-license.url}
       [2]: ${joypixels-license-appendix.url}
-'';
+    '';
+
+in
 
 stdenv.mkDerivation rec {
   pname = "joypixels";
   version = "6.0.0";
 
-  src = with systemSpecific; fetchurl {
+  src = assert !acceptLicense -> throwLicense;
+    with systemSpecific; fetchurl {
       inherit name;
       url = "https://cdn.joypixels.com/distributions/${systemTag}/font/${version}/${fontFile}";
       sha256 = {
@@ -85,7 +86,7 @@ stdenv.mkDerivation rec {
     homepage = "https://www.joypixels.com/fonts";
     license = let free-license = joypixels-free-license;
                   appendix = joypixels-license-appendix;
-      in {
+      in with systemSpecific; {
         spdxId = "LicenseRef-JoyPixels-Free-6.0-with-${capitalized}-Appendix";
         fullName = "${free-license.fullName} with ${appendix.fullName}";
         url = free-license.url;