fix custom_impl job

Author: newpavlov

.github/workflows/tests.yml

@@ -79,14 +79,15 @@ jobs:
         run: cargo test --features=std,sys_rng
 
   custom_impl:
-    name: Custom Implementations
+    name: Custom and External Implementations
     runs-on: ubuntu-24.04
     steps:
       - uses: actions/checkout@v6
       - uses: dtolnay/rust-toolchain@master
         with:
           toolchain: nightly
       - uses: Swatinem/rust-cache@v2
+      - run: cd custom_impl_test/
       - env:
           RUSTFLAGS: -Dwarnings --cfg getrandom_backend="custom"
         run: cargo test