[CI] Use new nicer override mechanism for ocamlPackages.elpi

Since NixOS/nixpkgs#374505

Author: proux01

.nix/config.nix

@@ -26,14 +26,16 @@
       coq.override.version = "master";
       stdlib.override.version = "master";
       coq-elpi.override.version = "master";
+      coq-elpi.override.elpi-version = "2.0.7";
       bignums.override.version = "master";
       paramcoq.override.version = "master";
       coqeal.job = false;  # broken in master, c.f. https://github.com/coq/coq/pull/19228
     };
-    "coq-master".ocamlPackages = { elpi.override.version = "2.0.7"; };
 
     "coq-8.20".coqPackages = mcHBcommon // {
       coq.override.version = "8.20";
+      coq-elpi.override.version = "v2.4.0";
+      coq-elpi.override.elpi-version = "2.0.7";
     };
 
     "coq-8.19".coqPackages = mcHBcommon // {

.nix/coq-nix-toolbox.nix

@@ -1 +1 @@
-"a24e6f4fb01d4ca0811dc8d16246cfe36b37ac52"
+"82da08ded8f5b913ab813ab079a698eff6ab70e2"