add missing line back

Signed-off-by: spacewander <[email protected]>

Author: spacewander

DEVELOPER.md

@@ -238,6 +238,8 @@ Before running any tests, please ensure the development dependencies are install
 make dev
 ```
 
+If Rust/Cargo doesn't work, try setting `export KONG_TEST_USER_CARGO_DISABLED=1` first.
+
 Kong relies on three test suites using the [busted](https://lunarmodules.github.io/busted/) testing library:
 
 * Unit tests