scripts/update: build required nix binary before updating packages

scripts/update

@@ -50,6 +50,7 @@ getPkgs() {
   # but since i use Import From Derivation along paths which i also want to query,
   # then i need to ensure those derivations are available for import.
   debug "creating requisite .drv store paths"
+  nix-instantiate -A nix "$NIX_FILES_TOP"
   nix-instantiate -A nixpkgs "$NIX_FILES_TOP"
   debug "querying attributes which match '$attrPrefix'"
   local attrs=$(nix-env -f "$NIX_FILES_TOP" --query --available --attr-path --no-name -A "$attrPrefix" --show-trace)