pin elixir version for nix builds again

(cherry picked from commit 975d43c2ae)

default.nix

@@ -1,6 +1,6 @@
 {
   lib,
-  beamPackages,
+  beam_nox,
   fetchFromGitHub,
   git,
   cmake,
@@ -11,6 +11,9 @@
 }:
 
 let
+  beamPackages = beam_nox.packages.erlang.extend (self: super: {
+    elixir = super.elixir_1_15;
+  });
   inherit (beamPackages) mixRelease buildMix;
 in
 mixRelease rec {