releng: move officialRelease to version.json

This was causing a few bits of suffering downstream, in particular, in
the NixOS module, which, after this change, can have the
`officialRelease` stuff in *it* completely deleted since we now have
correct defaulting in package.nix for it.

It also eliminates some automated editing of Nix files, which is
certainly always welcome to eliminate.

Fixes: https://git.lix.systems/lix-project/lix/issues/406
Change-Id: Id12f3018cff4633e379dbfcbe26b7bc84922bdaf

version.json

@@ -1,4 +1,5 @@
 {
     "version": "2.91.0-dev",
+    "official_release": false,
     "release_name": "TBA"
 }