overlays: add frappix fixups for nixos-unstable (sql_metadata, rq_1)

Building the frappix hosts on nixos-unstable 3ec6363 fails in frappix's
python overlay:

- sql_metadata 2.12.0 pins sqlparse<0.6, nixpkgs ships 0.6.0. frappe
  itself needs 0.6 (MAX_GROUPING_TOKENS), and sql_metadata 3.x needs
  sqlglot >= 30 which nixpkgs lacks, so relax the pin. frappe only feeds it
  SELECT statements; the only regressions on 0.6 are CREATE TABLE IF NOT
  EXISTS name extraction.
- rq 1.16.2 test_parse_connection fails against redis-py 8.1 (extra
  himport_registry kwarg). Test-only; disable it.

nix flake check does not catch these: it only evaluates hosts.

overlays/default.nix

@@ -10,5 +10,7 @@
     frappixPythonOverlay = inputs.frappix.pythonOverlay.x86_64-linux;
     frappixToolsOverlay = inputs.frappix.toolsOverlay.x86_64-linux;
     frappixAppsOverlay = import ../pkgs/frappix/overlay.nix;
+    # Must be applied after frappixPythonOverlay (overrides packages from it).
+    frappixFixupsOverlay = import ./frappix-fixups.nix;
   };
 }