Bump elpi versions

Author: gares

.nix/config.nix

@@ -30,10 +30,11 @@
       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 = "2.0.6"; };
+    "coq-master".ocamlPackages = { elpi.override.version = "#308"; };
 
     "coq-8.20".coqPackages = mcHBcommon // {
       coq.override.version = "8.20";
+      ocamlPackages = { elpi.override.version = "#308"; };
     };
 
     "coq-8.19".coqPackages = mcHBcommon // {