beam/buildMix: add appConfigPath arg

This is added for Mobilizon to be able to let it's dependencies read
it's config directory

pkgs/development/beam-modules/build-mix.nix

@@ -12,6 +12,9 @@
 , meta ? { }
 , enableDebugInfo ? false
 , mixEnv ? "prod"
+# A config directory that is considered for all the dependencies of an app, typically in $src/config/
+# This was initially added, as some of Mobilizon's dependencies need to access the config at build time.
+, appConfigPath ? null
 , ...
 }@attrs:
 
@@ -46,6 +49,14 @@ let
       runHook preConfigure
 
       ${./mix-configure-hook.sh}
+      ${lib.optionalString (!isNull appConfigPath)
+      # Due to https://hexdocs.pm/elixir/main/Config.html the config directory
+      # of a library seems to be not considered, as config is always
+      # application specific. So we can safely delete it.
+      ''
+        rm -rf config
+        cp -r ${appConfigPath} config
+      ''}
 
       runHook postConfigure
     '';