ConnectionPool middleware: fix a crash due to unimplemented behaviour

Structs don't implement Access behaviour, so this crashed. Tests didn't
catch it and I didn't test that part of the codepath. Very sorry

lib/pleroma/tesla/middleware/connection_pool.ex

@@ -32,7 +32,7 @@ def call(%Tesla.Env{url: url, opts: opts} = env, next, _) do
           {:ok, env} ->
             unless opts[:adapter][:body_as] == :chunks do
               ConnectionPool.release_conn(conn_pid)
-              {:ok, pop_in(env[:opts][:adapter][:conn])}
+              {:ok, pop_in(env.opts[:adapter][:conn])}
             else
               {:ok, env}
             end