use config_env() rather than Mix.env()

Author: scohen

apps/remote_control/config/config.exs

@@ -1,2 +1,2 @@
 import Config
-import_config("#{Mix.env()}.exs")
+import_config("#{config_env()}.exs")

apps/server/config/config.exs

@@ -5,4 +5,4 @@ config :snowflake,
   # First second of 2024
   epoch: 1_704_070_800_000
 
-import_config("#{Mix.env()}.exs")
+import_config("#{config_env()}.exs")