versioning: move variable

The variable is an internal implementation detail. Move it close to
where it's used.

modules/versioning.nix

@@ -22,8 +22,6 @@ let
     };
   };
 
-  version = config.nix-bitcoin.configVersion;
-
   # Sorted by increasing version numbers
   changes = [
     {
@@ -197,6 +195,8 @@ let
       '';
   };
 
+  version = config.nix-bitcoin.configVersion;
+
   incompatibleChanges = optionals
     (version != null && versionOlder lastChange)
     (builtins.filter (change: versionOlder change && (change.condition or true)) changes);