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

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