Run --install-pip on CI

.github/workflows/ci.yaml

@@ -105,6 +105,11 @@ jobs:
             test_exception_hierarchy test_glob test_iter test_list test_os test_pathlib
             test_py_compile test_set test_shutil test_sys test_unicode test_unittest test_venv
             test_zipimport test_importlib test_io
+      - if: runner.os == 'Linux'
+        name: check that --install-pip succeeds
+        run: |
+          mkdir site-packages
+          target/release/rustpython --install-pip -t site-packages
 
   lint:
     name: Check Rust code with rustfmt and clippy