beam/fetchMixDeps: allow adding patches

Needed to modify e.g. a few things in the `plausible` package to get it
running properly.

Changed files
+3
pkgs
development
beam-modules
+3
pkgs/development/beam-modules/fetch-mix-deps.nix
···
, mixEnv ? "prod"
, debug ? false
, meta ? { }
+
, patches ? []
, ...
}@attrs:
···
export REBAR_CACHE_DIR="$TMPDIR/rebar3.cache"
runHook postConfigure
'';
+
+
inherit patches;
dontBuild = true;