Use github action to set RUSTPYTHONPATH

.github/workflows/ci.yaml

@@ -69,9 +69,9 @@ jobs:
         run: pipenv run pytest -v
         working-directory: ./tests
       - name: run cpython tests
-        run: |
-          export RUSTPYTHONPATH=`pwd`/Lib
-          cargo run --release -- -m test -v
+        run: cargo run --release -- -m test -v
+        env:
+          RUSTPYTHONPATH: ${{ github.workspace }}/Lib
 
   format:
     name: Check Rust code with rustfmt and clippy