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

@@ -35,13 +35,15 @@
               dynamicZonesDir = "${bindData}/zones";
             in
             {
-              services.bind.zones.${testDomain} = {
+              bind = {
+                zones.${testDomain} = {
                   master = true;
                   file = "${dynamicZonesDir}/${testDomain}";
                   extraConfig = ''
                     allow-update { key rndc-key; };
                   '';
                 };
+              };
 
               systemd.services.bind.preStart = ''
                 # shellcheck disable=SC2211,SC1127