fix(config): Do not allow customizing cuttlefish env override prefix

We have quite a few EMQX_ prefixed variables used in bash scripts
having to meta-programming bash is not quite readable

bin/emqx

@@ -20,8 +20,8 @@ mkdir -p "$RUNNER_LOG_DIR"
 # Make sure data directory exists
 mkdir -p "$RUNNER_DATA_DIR"
 
-# cuttlefish try to read environment variables starting with "EMQX_", if not specified
-export CUTTLEFISH_ENV_OVERRIDE_PREFIX="${CUTTLEFISH_ENV_OVERRIDE_PREFIX:-EMQX_}"
+# cuttlefish try to read environment variables starting with "EMQX_"
+export CUTTLEFISH_ENV_OVERRIDE_PREFIX='EMQX_'
 
 relx_usage() {
     command="$1"