Fix wrong variable

compose/bin/setup-composer-auth

@@ -28,7 +28,7 @@ elif [ $USE_PYTHON3 ]; then
 
     if [ -f "$GLOBAL_AUTH" ]; then
         PUBLIC_KEY=$(python3 -c "$PY3_USER" 2>/dev/null < "$GLOBAL_AUTH")
-        PRIVATE_KEY=$(python3 -c "$PY3_USER" 2>/dev/null < "$GLOBAL_AUTH")
+        PRIVATE_KEY=$(python3 -c "$PY3_PASS" 2>/dev/null < "$GLOBAL_AUTH")
     fi
 
     if [ -z "$PUBLIC_KEY" ] || [ -z "$PRIVATE_KEY" ]; then