fix(flake): switch to github ref

Switch from a flake ref to a github ref as renovate cannot resolve flake
refs, and they are slightly impure.

This shows some issues with the previous patch to the bind module which
we fix.

flake-modules/tests.nix

@@ -35,14 +35,12 @@
               dynamicZonesDir = "${bindData}/zones";
             in
             {
-              bind = {
-                zones.${testDomain} = {
-                  master = true;
-                  file = "${dynamicZonesDir}/${testDomain}";
-                  extraConfig = ''
-                    allow-update { key rndc-key; };
-                  '';
-                };
+              services.bind.zones.${testDomain} = {
+                master = true;
+                file = "${dynamicZonesDir}/${testDomain}";
+                extraConfig = ''
+                  allow-update { key rndc-key; };
+                '';
               };
 
               systemd.services.bind.preStart = ''