fix custom_impl job

Author: newpavlov

.github/workflows/tests.yml

@@ -87,12 +87,13 @@ jobs:
         with:
           toolchain: nightly
       - uses: Swatinem/rust-cache@v2
-      - run: cd custom_impl_test/
       - env:
           RUSTFLAGS: -Dwarnings --cfg getrandom_backend="custom"
+        working_directory: custom_impl_test
         run: cargo test
       - env:
           RUSTFLAGS: -Dwarnings --cfg getrandom_backend="extern_impl"
+        working_directory: custom_impl_test
         run: cargo test
 
   ios: